Process for microbial transformation of steroids

Abstract

Novel microbial transformation process to selectively convert steroids with or without 17-alkyl side chains of from 2 to 10 carbon atoms, inclusive, to 3a.alpha.-H-4.alpha.-[3'-propionic acid]-7a.beta.-methylhexahydro-1,5-indanedione having the following structure: ##STR1## This compound can be used as an intermediate to make useful 19-nor steroids.

Description

BACKGROUND OF THE INVENTION

The transformation of steroids by microorganisms has been widely studied and documented. Apparently, the earliest such work was by Mamoli and Vercellone in 1937, Ber. 70, 470 and Ber. 70, 2079. They disclosed the reduction of 17-ketosteroids to 17.beta.-hydroxysteroids by fermenting yeast. More recently, Peterson and Murray disclosed the 11.alpha.-hydroxylation of progesterone with the fungus Rhizopus nigricans; see, U.S. Pat No. 2,602,769 (1952). Also recently, Kraychy et al. in U.S. Pat. No. 3,684,657 (1972) discloses the selective microbiological degradation of steroidal 17-alkyls by fermenting a steroid containing at least 8 carbons in the 17-alkyl side chain with Mycobacterium sp. NRRL B-3683 to prepare androst-4-ene-3,17-dione, androst-1,4-diene-3,17-dione, and 20.alpha.-hydroxymethyl-pregna-1,4-dien-3-one. Even more recently, Marsheck et al. in U.S. Pat. No. 3,759,791 (1973) disclose the selective microbiological preparation of androst-4-ene-3,17-dione by fermenting a steroid of the cholestane or stigmastane series containing at least 8 carbons in the 17-alkyl side chain with Mycobacterium sp. NRRL B-3805.

The compound produced by the process of the subject invention, 3a.alpha.-H-4.alpha.-[3'-propionic acid]-7a.beta.-methylhexahydro-1,5-indanedione, hereinafter referred to as Compound I, is a known compound described by Sih and Wang in their article "Mechanisms of steroid oxidation by microorganisms. II. Isolation and characterization of 3a.alpha.-H-4.alpha.-[3'-propionic acid]-7a.beta.-methylhexahydro-1,5-indanedione". J. Am. Chem. Soc., 85: 2135-2137 (1963). Sih and Wang described the production of Compound I as an intermediate in the degradation of androst-4-ene-3,17-dione by Nocardia restrictus.

Also percinent in the background of the subject invention is the article by Schubert, K., Bohme, K-H., and Horhold, C. entitled "Formation of low molecular degradation products from progesterone by microorganisms". Steroids 4, 581-586 (1964). These authors described the formation of tricyclic intermediates during the degradation of progesterone by Mycobacterium smegmatis.

The culture of the subject invention process, Mycobacterium fortuitum NRRL B-8129 is disclosed in Pending Application Ser. No. 632,635, filed on Nov. 17, 1975. The process conditions disclosed in Ser. No. 632,635 are not conducive to the production of Compound I of the subject application.

BRIEF SUMMARY OF THE INVENTION

A novel microbial transformation process using mutants which are characterized by their ability to selectively degrade steroids with or without 17-alkyl side chains of from 2 to 10 carbon atoms, inclusive, and accumulate predominantly Compound I in the fermentation beer.

These mutants can be obtained from sterol-degrading microorganisms of the following genera by using the mutation procedures disclosed herein or other mutation procedures: Arthrobacter, Bacillus, Brevibacterium, Corynebacterium, Mycobacterium, Nocardia, Protaminobacter, Serratia, and Streptomyces. A preferred genus is Mycobacterium. Exemplary species of this genus are M. phlei, M. Smegmatis, M. rhodochrous, M. mucosum, M. fortuitum, and M. butyricum. Specifically exemplified herein is a mutant microorganism, Mycobacterium fortuitum, NRRL B-8129, which is used to selectively degrade steroids with or without 17-alkyl chains containing from 2 to 10 carbon atoms, inclusive, to compound I. Examples of suitable steroids are sitosterols, cholesterol, stigmasterol, campesterol, and like steroids with 17-alkyl side chains of from 2 to 10 carbon atoms, inclusive, and androst-4-ene-3,17-dione, androsta-1,4-diene-3,17-dione, dehydroepiandrosterone, and testosterone. These steroid substrates can be in either the pure or crude form.

Also produced in the fermentation beer are lesser amounts of 3a.alpha.-H-4.alpha.-[3'-propanol]-7a.beta.-methylhexahydro-1,5-indandione hemiketal ##STR2## hereinafter referred to as Compound II, and traces of 3a.alpha.-H-4.alpha.-[3'-propanol]-5.alpha.-hydroxy-7a.beta.-methylhexahydro-1-indanone hemiacetal ##STR3## hereinafter referred to as Compound III; 3a.alpha.-H-4.alpha.-[3'-propionic acid]-5.alpha.-hydroxy-7a.beta.-methylhexahydro-1-indanone-.delta.-lactone ##STR4## hereinafter referred to as Compound IV; and, 3a.alpha.-H-4.alpha.-[3'-propanol]-5.alpha.-hydroxy-7a.beta.-methylhexahydro-1-indanone ##STR5## hereinafter referred to as Compound V.

DETAILED DESCRIPTION OF THE INVENTION

The Microorganisms

Mutants which are characterized by their ability to selectively degrade steroids with or without 17-alkyl side chains containing from 2 to 10 carbon atoms, inclusive, and accumulate predominantly compound I in the fermentation beer can be obtained by mutating sterol-degrading microorganisms of the following genera: Arthrobacter, Bacillus, Brevibacterium, Corynebacterium, Mycobacterium, Nocardia Protaminobacter, Serratia, and Streptomyces. Mycobacterium fortuitum, ATCC 6842, has been mutated, as disclosed herein, to give a laboratory mutant microorganism. The 1974 ATCC Catalogue discloses the following alongside the listing of ATCC 6842: "J. C. Cruz 2. Cold abscess. Acta Med. Rio de Janeiro 1:1 (1936). Medium 90 37C". M. fortuitum, ATCC 6842, degrades sterols-on-selectively to small molecular weight compounds, e.g., CO.sub.2 +H.sub.2 O. Thus, this microorganism is not suitable as a selective steroid degrader.

Mutation of M. fortuitum, ATCC 6842, using nitrosoguanidine has resulted in the production of a mutant which selectively degrades steroids with or without 17-alkyl side chains of from 2 to 10 carbon atoms, inclusive, to produce predominantly compound I in the fermentation beer. This mutant microorganism of M. fortuitum has been given the accession number NRRL B-8129, by the Northern Regional Research Laboratory, U.S. Department of Agriculture, Peoria, Illinois, U.S.A., where it has been deposited in the permanent collection. A subculture of this microorganism is freely available from this depository by request made thereto. It should be understood that the availability of the culture does not constitute a license to practice the subject invention in derogation of patent rights granted with the subject instrument by governmental action.

The morphology and drug sensitivities of M. fortuitum, NRRL B-8129, are indistinguishable from that of the parent M. fortuitum, ATCC 6842. Both M. fortuitum cultures are acid-fast non-motile, non-spore-forming bacilli belonging to the family Mycobacteriaceae of the order Actinomycetales. According to Runyon's classification (Runyon, E. H. 1959 Med. Clin. North America 43: 273) it is a nonchromogenic group IV mycobacterium, i.e., it grows rapidly at low temperature to produce nonpigmented colonies on relatively simple media.

M. fortuitum ATCC 6842 and M. fortuitum NRRL B-8129, are clearly distinguishable in their action on steroid molecules. As disclosed above, M. fortuitum ATCC 6842 is a degrader of steroids, whereas M. fortuitum NRRL B-8129 is a selective degrader. This property of M. fortuitum NRRL B-8129 makes it highly useful, as disclosed herein.

The mutation of M. fortuitum ATCC 6842 to give M. fortuitum NRRL B-8129 was accomplished by the use of nitrosoguanidine. The details of the procedure are described infra. Though mutation procedures are generally known in the art, there is no known art which teaches or even suggests the type of mutants, if any, which might be obtained by use of the subject mutation procedure. Also, though the mutation and transformation procedures, disclosed herein, are detailed for a Mycobacterium, it should be understood that similar or equivalent procedures can be used with microorganisms of the other genera, as disclosed herein.

The Transformation Process

The selective transformation of the subject invention can be effected in a growing culture of M. fortuitum NRRL B-8129 by either adding the selected steroid substrate to the culture during the incubation period, or incorporating it in the nutrient medium prior to inoculation. The steroid can be added singly or in combination with another steroid. The preferred, but not limiting, range of concentration of the steroid in the culture is about 0.1 to about 100 grams per liter. The culture is grown in a nutrient medium containing a carbon source, for example, as assimilable carbohydrate, and a nitrogen source, for example, an assimilable nitrogen compound or proteinaceous material. Preferred carbon sources include glucose, brown sugar, sucrose, glycerol, starch, cornstarch, lactose, dextrin, molasses, and the like. Preferred nitrogen sources include cornsteep liquor, yeast, autolyzed brewer's yeast with milk solids, soybean meal, cottonseed meal, cornmeal, milk solids, pancreatic digest of casein, fish meal, urea, distillers' solids, animal peptone liquors, meat and bone scraps, ammonium salts and the like. Combinations of these carbon and nitrogen sources can be used advantageously. Trace metals, for example, zinc, magnesium, manganese, cobalt, iron, and the like, need not be added to the fermentation media since tap water and unpurified ingredients are used as components of the medium. Prior to sterilization of the medium the pH is adjusted to a pH above 7, and preferrably to a pH in the range of about 7 to about 9. The pH adjustment can be made with a strong base, for example, NaOH, KOH, and the like.

A critical feature in the invention process is to maintain the pH above 7 during the fermentation in order to produce predominantly Compound I. This pH maintenance can be done by procedures well known in the art, for example, by use of CaCO.sub.3 or phosphate buffer in the medium, or by pH control with a base, for example, sodium hydroxide, ammonium hydroxide, and the like.

The transformation process can range from about 72 hours to 15 days. The incubation temperature during the transformation process can range from about 25.degree. C. to about 37.degree. C., with 31.degree. C. being preferred. The contents of the transformation vessel are aerated with sterilized air and agitated to facilitate growth of the microorganism, and, thus, enhance the effectiveness of the transformation process.

Upon completion of the transformation process, as evidenced by thin layer chromatography using silica gel plates (E. Merck, Darmstadt) and a solvent system consisting of 60:40:1 (by volume) ethyl acetate-cyclohexane-glacial acetic acid, the desired tranformed steroid is recovered by means well known in the art. For example, the fermentation (transformation) reaction mixture, including the fermentation liquor and cells, is adjusted to a pH of about 8 with a strong base, as above, or with a buffer solution, for example, a saturated solution of sodium bicarbonate and the like, and then extracted with a water-immiscible organic solvent for steroids. Suitable solvents are methylene chloride (preferred), chloroform, carbon tetrachloride, ethylene chloride, trichloroethylene, ether amyl acetate, benzene and the like. Compound I is obtained in good yield by adjusting the pH of the bicarbonate-treated reaction mixture to about 2 with a concentrated mineral acid solution, for example, a concentrated HCl solution. The reaction mixture then can be extracted, as above, to yield an extract containing Compound I. Upon the slow addition of cyclohexane to this extract there is obtained crystalline Compound I having a melting point range of 107.degree.-110.degree. C.

Compound I is useful as an intermediate in the chemical synthesis of useful steroids. For example, it can be converted to starting material for the process disclosed in U.S. Pat. No. 3,880,884 which discloses a process for the total synthesis of useful 19-nor steroids. This conversion to starting material can be done by procedures known in the art, e.g., by treatment with acetic anhydride and sodium acetate. See J.A.C.S. 85: 2135-2137.

The following examples are illustrative of the process of the subject invention but are not to be construed as limiting. All percentages are by weight and all solvent mixture proportions are by volume unless othewise noted.

EXAMPLE 1 Preparation of Mutant M. fortuitum NRRL B-8129 From M. fortuitum ATCC 6842

(a) Nitrosoquanidine Mutagenesis

Cells of M. fortuitum ATCC 6842 are grown at 28.degree. C. in the following sterile seed medium;

______________________________________Nutrient Broth (Difco) 8 g/literYeast Extract 1 g/literSodium Propionate 0.5 g/literDistilled Water, q.s. 1 liter______________________________________

The pH is adjusted to 7.0 with 1 N NaOH prior to sterilization at 121.degree. C. for 20 minutes.

The cells are grown to a density of about 5.times.10.sup.8 per ml, pelleted by centrifugation, and then washed with an equal volume of sterile 0.1 M sodium citrate, pH 5.6. Washed cells are resuspended in the same volume of citrate buffer, a sample removed for titering (cell count), and nitrosoguanidine added to a final concentration of 50 .mu.g/ml. The cell suspension is incubated at 37.degree. C. in a water bath for 30 minutes, after which a sample is again removed for titering and the remainder centrifuged down and washed with an equal volume of sterile 0.1 M potassium phosphate, pH 7.0. Finally, the cells are resuspended in a sterile minimal salts medium, minus a carbon source, consisting of the following:

______________________________________NH.sub.4 NO.sub.3 1.0 g/literK.sub.2 HPO.sub.4 0.25 g/literMgSO.sub.4 . 7H.sub.2 O 0.25 g/literNaCl 0.005 g/literFeSO.sub.4 . 7H.sub.2 O 0.001 g/literDistilled Water, q.s. 1 liter______________________________________

The pH is adjusted to 7.0 with 1 N HCl prior to sterilization at 121.degree. C. for 20 minutes. The cells are then plated out to select for mutants.

(b) Selection And Isolation Of Mutant M. fortuitum NRRL B-8129

Mutagenized cells, as described above, are diluted and spread onto plates containing a medium consisting of the following (modified from Fraser and Jerrel. 1963. J. Biol. Chem. 205: 291-295):

______________________________________Glycerol 10.0 g/literNa.sub.2 HPO.sub.4 8.4 g/literKH.sub.2 PO.sub.4 4.5 g/literNH.sub.4 Cl 2.0 g/literMgSO.sub.4 . 7H.sub.2 O 0.3 g/literFeCl.sub.3 . 6H.sub.2 O 0.05 g/literDistilled Water, q.s. 1 liter______________________________________

Agar (15 g/liter) is added, and the medium is autoclaved at 121.degree. C. for 30 minutes and then poured into sterile Petri plates.

Growth on this medium eliminates most nutritional auxotrophs produced by the mutagenesis procedure, e.g. cultures that require vitamins, growth factors, etc. in order to grow on chemically defined medium are eliminated. After incubation at 28.degree. C. for about 7 days, the resulting colonies are replicated to test plates suitable for selecting mutants and then back onto control plates containing the glycerol-based medium. The test plates are prepared as described by Peterson, G. E., H. L. Lewis and J. R. Davis. 1962. "Preparation of uniform dispersions of cholesterol and other water-insoluble carbon sources in agar media." J. Lipid Research 3: 275-276. The minimal salts medium in these plates is as described above in section (a) of Example 1. Agar (15 g/liter), and an appropriate carbon source (1.0 g/liter), such as sitosterol or androstenedione (AD), are added and the resulting suspension autoclaved for 30 minutes at 121.degree. C. The sterile, hot mixture is then poured into a sterile blender vessel, blended for several minutes, and then poured into sterile Petri plates. Foaming tends to be a problem in this procedure but can be reduced by blending when the mixture is hot and by flaming the surface of the molten agar plates. In this manner uniform dispersions of water-insoluble carbon sources are obtained which facilitates the preparation of very homogenous but opaque agar plates.

Colonies which grew on the control plates, but not on test plates containing AD as the sole carbon source, are purified by streaking onto nutrient agar plates. After growth at 28.degree. C., individual clones are picked from the nutrient agar plates with sterile toothpicks and retested by inoculating grided plate; containing AD as the carbon source. Purified isolates which still exhibit a phenotype different from the parental culture are then evaluated in shake flasks.

(c) Shake Flask Evaluation

Shake flasks (500 ml) contain 100 ml of biotransformation medium consisting of the following ingredients:

______________________________________Glycerol 10.0 g/literNa.sub.2 HPO.sub.4 8.4 g/literKH.sub.2 PO.sub.4 4.5 g/literNH.sub.4 Cl 2.0 g/literMgSO.sub.4 . 7H.sub.2 O 0.3 g/literFeCl.sub.3 . 6H.sub.2 O 0.05 g/literDistilled Water, q.s. 1 liter______________________________________

Soyflour (1 g/liter) is blended into the medium and then sitosterol (10 g/liter) is also blended into the medium. After the flasks are autoclaved for 20 minutes at 121.degree. C., they are cooled to 28.degree. C. and then inoculated with 10 ml of seed growth prepared as follows:

The purified isolates from part (b) are grown on agar slants at 28.degree. C. A loop of cells taken from a slant is used to inoculate a 500-ml flask containing 100 ml of sterile seed medium consisting of the following ingredients:

______________________________________Nutrient Broth (Difco) 8 g/literYeast Extract 1 g/literGlycerol 5 g/literDistilled Water, q.s. 1 liter______________________________________

The pH is adjusted to 7.0 with 1 N NaOH prior to autoclaving the flasks at 121.degree. C. for 20 minutes. The seed flasks are incubated at 28.degree. C. for 72 hours.

As disclosed above, 10 ml of seed growth is then used to inoculate each 500-ml flask containing 100 ml of sterile transformation medium. The flasks are then incubated at 28.degree. C. to 30.degree. C. on a rotary shaker and sampled at various intervals. Ten ml samples are removed and extracted by shaking with 3 volumes of methylene chloride. Portions of the extracts are analyzed by thin layer chromatography using silica gel and the solvent system described above, i.e., 2:3 (by volume) ethyl acetate-cyclohexane, and by gas-liquid chromatography. Evidence of the presence of compounds II and V confirms the selective degradation of sitosterol by the novel mutant produced from the parent M. fortuitum ATCC 6842.

EXAMPLE 2 Transformation of Sitosterol to Compound 1

The medium used is the same as in Example 1 (c), except that it is adjusted to a pH of about 7.5 with 4 N NaOH. This medium is sterilized by heating 30 minutes at 121.degree. C., whereupon it is cooled to 28.degree. C. and then inoculated with 10 parts of a seed culture of the mutant M. fortuitum NRRL B-8129, prepared as described in Example 1 (c). The inoculated mixture is incubated at 28.degree. C. for 168 hours with agitation to promote submerged growth. As required, the pH is maintained between 7 and 9 by the addition of NaOH. Progress of the biotransformation is followed by acidifying a sample of the reaction mixture to pH 2 with a concentrated HCl solution, extracting with four volumes of methylene chloride, and chromatographing the extract on thin layer silica gel plates in a solvent system consisting of 60:40:1 (by volume) ethyl acetate-cyclohexane-glacial acetic acid.

Upon completion of the biotransformation the products formed are isolated by the following procedure. The reaction mixture is adjusted to pH 8 by the addition of a saturated solution of sodium bicarbonate and then extracted with methylene chloride. This extract, which contains Compounds II.fwdarw.V, but not Compound I, is filtered through diatomaceous earth and the filtrate is distilled to dryness. The residue from the filtrate is taken up in chloroform and chromatographed on silica gel, using Skellysolve B and mixtures thereof with increasing amounts of ethyl acetate as developing solvent. This procedure elutes Compounds II, III and IV, but not V, which is taken off of the column with 5% methanol in ethyl acetate. The order of elution of these compounds is II.fwdarw.III.fwdarw.IV.fwdarw.V. The individual products are obtained by pooling the appropriate fractions, as detected by thin layer chromatography, and evaporating these to dryness. Upon recrystallization from ethyl acetate, compound II, a 1:1 mixture of two epimers, melts at 100.degree.-112.degree. C.; Compound IV melts at 122.degree.-124.degree. C.; and Compound V melts at 148.degree.-150.degree. C. Compound I is obtained in good yield by adjusting the pH of the bicarbonate-treated reaction mixture to 2 with a concentrated HCl solution, extracting again with chloroform or methylene chloride, and crystallizing out the product by the slow addition of cyclohexane. Compound I melts at 107.degree.-110.degree. C.

EXAMPLE 3

By substituting cholesterol for sitosterol in Example 2 there is obtained Compound I.

EXAMPLE 4

By substituting stigmasterol in Example 2 for sitosterol there is obtained Compound I.

EXAMPLE 5

By substituting campesterol for sitosterol in Example 2 there is obtained Compound I.

EXAMPLE 6

By adding a combination of any of the sterols in Examples 2-5, in addition to sitosterol, or in place of sitosterol, in Example 2 there is obtained Compound I.

EXAMPLE 7

By substituting a sterol-degrading microorganism from the genera Arthrobacter, Bacillus, Brevibacterium, Corynebacterium, Nocardia, Protaminobacter, Serratia, and Streptomyces, in Example 1 for Mycobacterium furtuitum ATCC 6842 there are obtained mutant microorganisms which are characterized by their ability to selectively degrade steroids with or without a 17-alkyl side chain of from 2 to 10 carbon atoms, inclusive, and accumulate predominantly Compound I in the fermentation beer.

EXAMPLE 8

By substituting the mutants obtained in Example 7 for M. fortuitum NRRL B-8129 in Examples 2-6, there is obtained Compound I.

EXAMPLE 9

By substituting a sterol-degrading microorganism selected from the group consisting of Mycobacterium phlei, M. smegmatis, M. rhodochrous, M. mucosum, and M. butyricum for M. fortuitum ATCC 6842 in Example 1 there are obtained mutant microorganisms which are characterized by their ability to selectively degrade steroids with or without a 17-alkyl side chain of from 2 to 10 carbon atoms, inclusive, and accumulate predominantly Compound I in the fermentation beer.

EXAMPLE 10

By substituting the mutants obtained in Example 9 for M. fortuitum NRRL B-8129 in Examples 2-6, there is obtained Compound I.

EXAMPLE 11

By substituting a compound selected from the group consisting of androst-4-ene-3,17-dione, androsta-1,4-diene-3,17-dione, dehydroepiandrosterone, and testosterone for sitosterol in Example 2 there is obtained Compound I.

EXAMPLE 12

By substituting a combination of two or more compounds selected from the group consisting of sitosterol, cholesterol, stigmasterol, androst-4-ene-3,17-dione, androsta-1,4-diene-3,17-dione, dehydroepiandrosterone, and testosterone for sitosterol in Example 2 there is obtained Compound I.

EXAMPLE 13

By substituting the mutants obtained in Example 7 for M. fortuitum NRRL B-8129 in Examples 11 and 12, there is obtained Compound I.

EXAMPLE 14

By substituting the mutants obtained in Example 9 for M. fortuitum NRRL B-8129 in Examples 11 and 12, there is obtained Compound I.

Claims

1. A process for preparing a compound, of the formula ##STR6## which comprises cultivating a mutant microorganism selected from the group consisting of Arthrobacter, Bacillus, Brevibacterium, Corynebacterium, Potaminobacter, Serratia, and Streptomyces, said mutant being characterized by its ability to selectively degrade steroids with or without a 17-alkyl side chain of from 2 to 10 carbon atoms, inclusive, and accumulate predominantly 3a.alpha.-H-4.alpha.-[3'-propionic acid]-7a.beta.-methylhexahydro-1,5-indanedione in the fermentation beer, in an aqueous nutrient medium at a pH of about 7 to about 9 under aerobic conditions in the presence of a steroid with or without a 17-alkyl side chain containing from 2 to 10 carbon atoms, inclusive.

2. A process, according to claim 1, wherein said mutant microorganism is cultivated in an aqueous nutrient medium under aerobic conditions in the presence of a mixture of two or more steroids.

3. A process, according to claim 1, wherein said steroid is selected from the group consisting of sitosterol, cholesterol, stigmasterol, campesterol, androst-4-ene-3,17-dione, androsta-1,4-diene-3,17-dione, dehydroepiandrosterone, and testosterone.

4. A process, according to claim 2, wherein said steroid mixture is selected from the group consisting of sitosterol, cholesterol, stigmasterol, campesterol, androst-4-ene 3,17-dione, androsta-1,4-diene-3,17-dione, dehydroepiandrosterone, and testosterone.

5. A process for preparing a fermentation beer, ##STR7## containing predominantly the compound of the formula ##STR8## which comprises cultivating a mutant microorganism selected from the group consisting of Arthrobacter, Bacillus, Brevibacterium, Corynebacterium, Protaminobacter, Serratia, and Streptomyces, said mutant being characterized by its ability to selectively degrade steroids with or without a 17-alkyl side chain of from 2 to 10 carbon atoms, inclusive, and accumulate predominantly 3a.alpha.-H-4.alpha.-[3'-propionic acid]-7a.beta.-methylhexahydro-1,5-indanedione in the fermentation beer, in an aqueous nutrient medium at a pH of about 7 to about 9 under aerobic conditions in the presence of a steroid with or without a 17-alkyl side chain containing from 2 to 10 carbon atoms, inclusive.

6. A process, according to claim 5, wherein said mutant microorganism is cultivated in an aqueous nutrient medium under aerobic conditions in the presence of a mixture of two or more steroids.

7. A process, according to claim 5, wherein said steroid is selected from the group consisting of sitosterol, cholesterol, stigmasterol, campesterol, androst-4-ene-3,17-dione, androsta-1,4-diene-3,17-dione, dehydroepiandrosterone, and testosterone.

8. A process, according to claim 6, wherein said steroid mixture is selected from the group consisting of sitosterol, cholesterol, stigmasterol, campesterol, androst-4-ene-3,17-dione, androsta-1,4-diene-3,17-dione, dehydroepiandrosterone, and testosterone.