Physiology of cortexolone biotransformation by fungi

The hydroxylation of cortexolone at the 11-position has been investigated using a range of isolates from filamentous fungi. Fungi obtained both from natural sources and culture collections have been screened for cortexolone and/or progesterone hydroxylation. About 75% of isolates from Agave were active modifiers of cortexolone, but ll(3-hydroxycortexolone and lla-hydroxycortexolone were not produced in large amounts relative to other products, and so these organisms were not chosen for further study. In contrast, several organisms obtained from culture collections, whilst hydroxylating the substrate at the 11-position produced fewer by-products. Germinating spores, mycelium and pellets of fungal biomass all showed hydroxylating activity. Cortexolone is transformed to both 11a- and 11(3- hydroxylated products, whereas progesterone is preferentially converted by 11ahydroxylation, presumably due to steric hinderance of 11 (3-hydroxylases. Phascolomyces articulosus and Absidia spinosa were discovered to be novel in producing both 11(3- hydroxycortexolone and lla-hydroxycortexolone. Comparative spore germination and vegetative cell transformation studies have been performed using Absidia spinosa, Cunninghamella blakesleeana and Cunningham ella elegans. Cortexolone bioconversion was found to correlate with germ tube production in Absidia spinosa and Cunninghamella blakesleeana. In all three organisms initial 11(3- hydroxycortexolone production always preceded lla-hydroxycortexolone production. The 11(3- and 11a- activities occurred during growth, but can function when growth has ceased. Pellet formation appeared to promote steroid metabolism in conditions of glucose exhaustion. In Absidia spinosa, the cessation of growth promoted the secondary transformation of the primary transformation products. Primary transformation enzymes can still be operative in the decline phase, but are superceded by other reactions. 11(3- hydroxycortexolone was the major source of secondary transformation products, l l a - hydroxycortexolone being metabolized in decline phase. Studies during vegetative growth of Absidia spinosa revealed several novel findings. Cortexolone and 11 (3-hydroxycortexolone do not inhibit the fungal growth rate, but energy may be required to remove them from the cell. Both cortexolone and 11 (3-hydroxycortexolone can be metabolized in exponential growth. Both 11(3- and 11a- hydroxylation products were produced very quickly in response to cortexolone addition. Both 11(3- and 1 la-activities have closely linked but distinct properties. Both enzymes are active over a broad extracellular pH and temperature range. Cortexolone bioconversion occurred at 35°C, in the absence of growth. The enzymes are differentially affected by antifungal agents. Investigations on the feasibility of immobilizing fungal spores into Chromosorb W beads were successful and are discussed. Further experimentation was not continued as studies with non-immobilized cells seemed to be more appropriate before exploiting the cells on immobilized supports. The preparation of cell-free extracts is described but successful retention of biotransformation activity was not recorded due to the instability of the enzymes involved in steroid bioconversions once cell breakage was performed. 11-Hydroxylating activity could fulfil mainly a detoxification role, providing transformation, rapid excretion or degradation of hydrophobic steroid molecules.