A plasma factor participates in the cryoactivation of plasma inactive renin

Plasma inactive renin (IR) can be cold activated (-4 degrees C). This process is called cryoactivation. The mechanism of cryoactivation is not yet elucidated. To investigate the mechanism of cryoactivation, we initially determine the isoelectric point (pI) of plasma active renin (AR) and IR by means of isoelectric focusing. The electrofocusing was performed in a LKB 8100-1 column containing a sucrose gradient and pH 3.5-10 ampholine under 4 degrees C. We found that the pI of AR was 5.0 and 3.9 whereas the pI of IR was 5.6. By using pH 4-6 ampholine, we separated IR from AR and added all fractions containing only IR to each of the electrofocusing column eluates (pH 4-6 ampholine). We found there was no increase in renin activity. If each fraction of electrofocusing eluate (after pH 4-6 ampholine) was cryoactivated, without adding the IR containing fractions, a small increase in angiotensin I (AI) generation was seen in the fractions of pH 4.9-5.1. However, when eluates from the electrofocusing column using pH 3.5-10 ampholine were added to fractions containing only inactive renin, and cryoactivation was subsequently performed, a peak increase in AI generation was found in the fractions corresponding to pH 4.9-5.1. Therefore, cryoactivation may expose certain enzyme(s) which have the pI of 4.9-5.1 and can activate inactive renin. We further found that these fractions could split the synthetic substrate S-2302. Such an activity may be produced by plasmin or related enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)