High levels of active 40 000-dalton renin in mouse saliva, but no evidence of inactive or high molecular weight forms

Abstract

The presence of renin in parasympathetically elicited mouse saliva was demonstrated by using both the antibody trapping method, which measures renin's enzymatic activity, and a direct radioimmunoassay, which detects the renin molecule by its antigenic properties. Crossed immunoelectrophoresis of saliva samples using an antiserum elicited against pure submaxillary renin showed only one precipitation line, indicating the presence of only one form of renin. The position of the line was similar to that found when submaxillary gland extract was subjected to crossed immunoelectrophoresis. Tandem crossed immunoelectrophoresis showed complete identity between antigenic determinants in submaxillary and salivary renin. An apparent molecular weight of about 35 000 and 38 000 was found when saliva samples were subjected to gel filtration on Ultrogel AcA 44 and Sephadex G-100, respectively. No high molecular weight forms were present and no inactive forms could be demonstrated after limited pepsin or trypsin proteolysis. The specific enzymatic activity of renin in pilocarpine saliva was 0.37 Goldblatt Units (G.U.) · μg −1, which is identical to that of pure submaxillary gland renin (0.41 G.U. · μg −1) and to that of the storage form of renin in the submaxillary gland (0.4 G.U. · μg −1). An identical K m value was found for salivary renin, 1.01 μM, and for pure submaxillary renin, 0.98 μM. It is concluded that renin in pilocarpine-elicited saliva is similar to the storage form of renin in the submaxillary gland with respect to molecular weight, enzymatic and immunological properties.