Microbial hydroxylations: VII. Kinetic studies on the hydroxylation of 19-norsteroids bY Curvularia lunata

Abstract

19-Nortestosterone induces hydroxylases for its own metabolism by Curvularia lunata NRRL 2380, optimum induction times of 1–3 h being indicated. Comparison of product spate and hydroxylation rates for vegetative cell culture incubations and for cell-free enzyme systems suggests that the vegetative cell system may be used for study of the initial kinetics of hydroxylation of steroid substrates without interference of endogenous cellular metabolism. Double reciprocal plots of kinetic data establish that the three major modes of transformation of 19-nortestosterone (to the 10β-, the 11β-, and the 14α-monohydroxylated derivatives) have identical apparent Michaelis constants ( K m 0.34 mM). Mathematical treatment of a model system supports identical K m values as found experimentally. Arguments based on these kinetics are presented which suggest that the 19-norsteroid hydroxylase system of C. lunata NRRL 2380 not be three individual hydroxylases acting independently but rather be a single hydroxylase of more generalized function.