Partial characterization of placental 3 beta-hydroxysteroid dehydrogenase (EC 1.1.1.145), delta 4-5isomerase (EC 5.3.3.1) in human term placental mitochondria.

A partial characterization of human term placental 3 beta-HSDH in mitochondria is reported. Apparent KM of pregnenolone: 70 nM. A dose-dependent stimulation of 3 beta-HSDH by NAD+ or NADP+ was observed in the range from 10(-6) to 10(-3) M (KM value of NAD+: 20 microM). At equimolar concentrations NAD+ is more than 10-fold as effective a cofactor of the 3 beta-HSDH than NADP+. pH optimum: 9.5 (glycine-NaOH buffer). Temperature optimum 40-45 degrees C. A rapid loss of 3 beta-HSDH activity was found after preincubation of the enzyme at 37 degrees C after 30 min; less than 50% of initial enzyme activity is present. No inhibition was obtained by Mg2+, Ca2+ Sr2+ and Ba2+ (1-100 mM). A strong inhibition was achieved with 1 mM Zn2+, Cd2+, Cu2+ and 10 mM and 100 mM Fe2+, Mn2+, Co2+ and Ni2+.