Crystal data for tonin, an enzyme involved in the formation of angiotensin II

Tonin is an enzyme isolated from the submaxillary glands of old rats. This enzyme is involved in the direct conversion of angiotensinogen or the tetradecapeptide renin substrate to the octapeptide angiotensin II, the agent implicated in hypertension. Large well-formed crystals of tonin were grown from 2.5 m-(NH4)2SO4, 0.01 m-phosphate buffer (pH 6.2) and 1% methylpentanediol. The crystals are trigonal, space group P3121 (P3221) with unit cell dimensions: a = b = 94 Å, c = 66 Å, γ = 120°. The value of Vm = 2.6 Å3/dalton corresponds to one molecule of molecular weight 32,000 per asymmetric unit. Definitive classification of tonin into one of the four proteolytic enzyme classes (carboxyl protease, serine protease, thiol protease or metalloprotease) has not been firmly established.