β-Oxidation cycle reactions, which are key stages in the metabolism of fatty acids in eucaryotic cells and in processes with a significant role in the degradation of acids used by microbes as a carbon source, have also found application in biotransformations. One of the major advantages of biotransformations based on the β-oxidation cycle is the possibility to transform a substrate in a series of reactions catalyzed by a number of enzymes. It allows the use of sterols as a substrate base in the production of natural steroid compounds and their analogues. This route also leads to biologically active compounds of therapeutic significance. Transformations of natural substrates via β-oxidation are the core part of the synthetic routes of natural flavors used as food additives. Stereoselectivity of the enzymes catalyzing the stages of dehydrogenation and addition of a water molecule to the double bond also finds application in the synthesis of chiral biologically active compounds, including medicines. Recent advances in genetic, metabolic engineering, methods for the enhancement of bioprocess productivity and the selectivity of target reactions are also described.

Research on steroid biooxidation is being pursued for development of processes exploitable by the pharmaceutical industry, and also for the purpose of preparation of potentially useful steroid analogues which are otherwise inaccessible. The use of enzymes provides high regio- and stereoselectivity of the reaction to be performed. In this work a review of recent and important findings related to microbial and enzymatic biotransformations of steroidal compounds including hydroxylation, Baeyer-Villiger oxidation, dehydrogenation and sterol side-chain cleavage is presented, with emphasis on processes of practical biotechnological importance. Although some of these bioconversions are well-established, efforts are ongoing in order to increase their efficiency as well as to discover novel steroidal compounds and acquire recombinant microorganisms capable of performing desired steroid biotransformations.

The course of transformation of DHEA, androstenediol and epiandrosterone in Mortierella isabellinaAM212 culture was investigated. The mentioned substrates underwent effective hydroxylation; 5-ene substrates – DHEA and androstenediol – were transformed into a mixture of 7α- and 7β- allyl alcohols, while epiandrosterone was converted into 7α- (mainly), 11α- and 9α- monohydroxy derivatives. Ketoconazole and cycloheximide inhibition studies suggest the presence of constitutive and substrate-induced hydroxylases in M. isabellina. On the basis of time course analysis of the hydroxylation of DHEA and androstenediol, the oxidation of allyl C₇-H_α and C₇-H_β bonds by the same enzyme is a reasonable assumption.