THE MECHANISM OF THE MICROBIAL HYDROXYLATION OF STEROIDS PART 1, THE C-21 HYDROXYLATION OF PROGESTERONE BY ASPERGILLUS NIGER ATCC 9142

HERBERT L. HOLLAND and BARBARA J. AURET. Can. J. Chem. 53,845 (1975). The mechanism of the C-21 hydroxylation of progesterone ( la) by Aspergillus niger ATCC 9142 to give 11deoxycorticosterone ( lb) has been studied by the use of progesterone derivatives and of C-21 deuterium labelled progesterones. The requirement of the C-21 hydroxylase system for a C-20 carbonyl group is demonstrated and the possiblity of the involvement of this group, in the C-20,21 enol form, in the C-21 hydroxylation reaction has been discussed. However, on the basis of the observed deuterium isotope effect (KHIKD = 1.25), a mechanism for the microbial C-21 hydroxylation reaction is proposed in which the rate-determining step is the direct insertion of oxygen into a C-21 carbon-hydrogen bond and not one involving enolization of the C-20 carbonyl. In addition, C-1 l a and C-15P hydroxylation of both 20aand 20P-hydroxypregn-4-ene-3-one (2a and 2b) by A. niger has been observed.