Renin-Renin Substrate Kinetic Constants in the Plasma of Normal and Estrogen-Treated Humans

The renin-renin substrate Michaelis constant (Km) and maximal velocity (Vmax) were determined in the plasmas of normal subjects. The mean Km was 0.7 microgram/ml. Under these conditions, the in vitro reaction of renin with physiologic concentrations of renin substrate will proceed at only 70% of its maximal velocity. Following estrogen administration, Km doubled to a value of 1.3 microgram/ml. Vmas increased by 81%. Analysis of the changes induced in the in vitro reaction velocity demonstrated that estrogen-induced acceleration of the renin reaction is dependent upon both an increase in renin substrate concentration as well as an increase in Vmas. The latter appears to be quantitatively more important. These findings suggest the emergence of modifying factors in the renin-renin substrate interaction following estrogen administration.