Formation of angiotensin II by tonin from partially purified human angiotensinogen

The renin substrate (angiotensinogen) has been purified from outdated human blood bank plasma. A 100-fold purification was achieved by ammonium sulphate protein fractionation and four successive chromatographic procedures. We show that tonin, a serine protease enzyme found in submaxillary glands of the rat, cleaves the human plasma angiotensinogen, devoid of tonin inhibiting factor(s), at a pH optimum of 5--5.5. It generates a pressor substance that was identified as angiotensin (A) II. The rate of cleavage of the human angiotensinogen preparation by 1 nmol of renin or tonin was calculated to be 1320 nmol AI/h for renin and 26 nmol AII/h for tonin.