The mechanism of the acid activation of rabbit uterine renin.

Besides active renin an inactive form of renin could be demonstrated in uterine tissue. On gel filtration it was eluted as a molecule of slightly higher molecular weight than active renin, and it could be irreversibly activated by acidification at 37 degrees C. The activation had a pH optimum between pH 3.8 and pH 5.3. Acid activated uterine renin was found identical to active uterine renin by 1) the formation of angiotensin I with time after addition of rat substrate, 2) the pressor response in the rat, 3) neutralization by antirenin and 4) similar Michaelian constants. Repeated freezing and thawing, acidification at 4 degrees C and dialysis against 4 mol/l NaCl did not give any activation. A lower rate of activation of diluted samples and activation by trypsin at pH 7.4 suggest that proteolytic enzymes are involved in the activation.