Abstract
We have previously reported on the differences in physical and chemical characteristics between the high-molecular weight renin substrate (HMS>150, 000 daltons) and the normal substrate (NMS= 60, 000).
In this study, the kinetic constants were determined in both HMS and NMS which were prepared by gel exclusion chromatography from the plasma of pregnant women at term.
Renin substrate (angiotensinogen) levels were expressed by radioimmunoassay of angiotensin I after incubation of samples with added semi-purified human kidney renin in the presence of angiotensinase inhibitors.
The kinetic constants (Km and Vmax) were determined by the method of Lineweaver-Burk plots and also the method of Wilkinson. The Km for the HMS was 1.79μg angiotensin I equivalents (AIeq)/ml and the Vmax= 41.2 ngAleq/ml/h, and the Km for the NMS was 3.52μgAleq/ml and the Vmax = 138 ng/ml/h.
When adding small amounts of the HMS to the NMS, the production of angiotensin I was found to increase more than that in the NMS alone.
It was also observed that the renin substrate reactivities of the plasma of pregnant women, which contained small amounts of the HMS, were higher than that found in the plasma of normotensive women not taking oral contraceptives.
It is suggested that the existence of small amounts of the HMS may therefore contribute to the elevation in blood pressure under the influence of estrogens.
