Steroid Biotransformation

Steroids are naturally occurring compounds  that have potent properties as hormones. They are also important therapeutic agents.

Steroids are highly diverse and are produced in the complex cyclopentano-perhydrophenanthrene system.  

In the body, a number of steroids are manufactured by the adrenal cortex and include cortisol, cortisone and corticosterone. There are also sex hormones to be considered such as testosterone which is the male sex hormone whilst female sex hormones include estradiol and estrogen as well as the hormone required for the regulation of the menstrual cycle, progesterone.

An important component of cell membranes is cholesterol and this is a substance which has been linked to heart disease and other nutritional conditions if present in excess or distributed incorrectly. We must all be familiar with the concept of hypercholesterolemia which is when our blood cholesterol levels are too high. 

The pharmaceutical industry relies heavily on steroidal biotransformation as a way of creating new drugs related to these hormones as well as to other natural compounds. The steroids in particular are used in the management of cancers, osteoporosis and certain viral infections such as AIDS. A very powerful anti-inflammatory drug, prednisolone is manufactured successfully using immobilised cells for example.

Currently, about 300 steroid drugs are recognised, and this number is growing.

A number of reactions are employed in transforming steroids. They include various types of oxidation:

• hydroxylation
• dehydrogenation 
• epoxidation

These reactions are catalysed in the body via long biochemical sequences involving specific enzymes. These are hydroxylases, dehydrogenases and epoxidases.

Some microbes possess the enzyme steroid-Δ¹-dehydrogenase which is used to convert cortisol to prednisolone.

Biotransformation Reactions

Immobilised cells have long been used for the manufacture of a variety of compounds from simple substrates. The manufacture of steroids is no different. From the 1930s steroids were being synthesized in batch fermentation cultures using various microorganisms.

Immobilization of living cells helps to stabilise it and protect it to some extent from damaging and deleterious reactions. In most cases the reaction rates that are needed for steroid synthesis are improved because of the immobilization conditions. Immobilization however does reduce the overall total activity of an enzyme because of restrictions in substrate access at the site of catalysis.

A good example of such a benefit was the immobilization of Arthrobacter simplex cells in a polyacrylamide gel (Ohlson et al., 1978) and in calcium alginate (Ohlson et al., 1979). These gels are highly porous to nutrients but effective at trapping living bacterial cells.  The enzyme steroid-Δ¹-dehydrogenase was studied because of its vital importance in the manufacture of prednisolone which comes from dehydrogenating cortisol. Cortisol too is an inducer of the enzyme at a genetic regulation level and may well have helped in the production of enzyme by those cells and not just through overall increased cell generation.

The process of immobilization in polyacrylamide gel dropped the level of enzyme activity by 60% compared to its activity in the free bacteria. However, the living cells benefitted from defined nutrient supplies whilst they were immobilised which raised their growth rate and also improved enzyme activity. A good nutrient medium was 0.5% peptone with 0.2% glucose.

The immobilised cells were also frozen and only lost a small fraction of enzyme activity after 4 months of freezing when they were reconstituted.

Similar effects from substrate induction have been noticed with other steroid transforming systems. The bacteria Rhizopus nigricans contains the enzyme  progesterone 11α-hydroxylase which is induced by its substrate progesterone. Production of the enzyme was enhanced in a fermentation by the presence of not only its main substrate but also by raising the oxygen content in the nutrient medium (Hanish et al., 1980). 

References

Hanish, W. H., Dunnill, P., & Lilly, M. D. (1980). Optimization of the production of progesterone 11α‐hydroxylase by Rhizopus nigricans. Biotechnology and Bioengineering, 22(3), pp. 555-570.

Ohlson, S., Larsson, P. O., & Mosbach, K. (1978). Steroid transformation by activated living immobilized Arthrobacter simplex cells. Biotechnology and Bioengineering, 20(8), pp. 1267-1284.

Ohlson, S., Larsson, P. O., & Mosbach, K. (1979). Steroid transformation by living cells immobilized in calcium alginate. European Journal of Applied Microbiology and Biotechnology, 7(2), pp. 103-110.