Serum renin substrate

The site of formation of renin is not known, although the indirect and circumstantial evidence favors slightly the juxtaglomerular apparatus rather than the tubules as a source (18). Crude renin, however, is extracted readily from renal cortex by saline extraction, acidification, and precipitation with ammonium sulfate and sodium chloride. Other active protein substances are likewise extracted, and their separation from renin is often a matter of considerable difficulty. The renin substrate, an arglobulin, is found in blood serum and is probably formed by the liver. It can be easily salted out of beef serum as a crude preparation. 

Plentl, Page, and Davis (142) found renin substrate from hog serum to be soluble in 0.45 saturated ammonium sulfate solution (1.85 M) and precipitated at 0.51 saturation (2.10 M). An electrophoretic analysis of hog serum showed that renin substrate was identical to, or moved with the same electrophoretic mobility as, a2-globulm. In a later paper, Plentl and Page (140) suggested the raction from hog serum precipitated (at pH 6.0) between 0.36 (1.5 M) and 0.49 (2.0 M) saturation with ammonium sulfate for the production of angiotonin. 

Pepsin hydrolyzes renin substrate with the formation of a pressor substance which was named pepsitensin (28). Furthermore, pressor substances were obtained by peptic digestion of casein, fibrin, serum albumin, and ovalbumin. No pressor substance was liberated when gelatin was treated with pepsin, which probably indicates that aromatic amino acids are essential constituents of pressor peptides. 

The activities of renin in plasma and serum are measured in the diagnosis of hypertension. The natural substrate for renin is angiotensin. Several synthetic peptides have been used in renin assays. Recently, Nakamura-lmajo et al. (1992) used N-(2-pyridyl)glycine (pg) as a fluorescent tag on a nonapeptide. 

Both the REF and renin are kidney enzymes which act on a plasma protein substrate. However, their different chemical and physiologic properties militate against their being the same substance. Thus, renin is stable in acid solutions (pH 2.5), while the REF is inactivated at pH 5.0 Moreover, intraperitoneal administration to polycythemic assay mice of 3.7 Goldblatt units of purified renin, incubated for 60 minutes in saline or in diaHyzed serum, did not result in stimulation of erybhropoiesis (Fig. lO). 

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