1. Renin-like enzymes have been isolated from aorta, plasma and kidney tissue of the rat. The enzymes have been compared with respect to pH optima, molecular weight and isoelectric points.

2. Two distinct molecular-weight forms were isolated from plasma. The high-molecular-weight enzyme (mol. wt. > 150 000) appeared to be homogeneous with respect to isoelectric point (pI = 5.3), whereas the dominant lower-molecular-weight form (mol. wt. 44 000) demonstrated isoelectric heterogeneity.

3. The renin-like enzyme isolated from aortic tissue has mol. wt. 44 000 and also demonstrated isoelectric heterogeneity (pI = 5.0, 5.3). Plasma renin was significantly reduced after bilateral nephrectomy (30 h); however, the activity and relative proportions of the two aortic enzymes were not significantly altered.

4. Renin, isolated from kidney homogenates, either in the presence or absence of a variety of proteolytic enzyme inhibitors, had a significantly lower molecular weight (38 000) than the plasma enzyme but demonstrated a similar pattern of isoelectric heterogeneity.

5. in contrast with renin extracted from kidney, approximately 30% of the renin released from kidney cortical slices into Krebs-Ringer bicarbonate had mol. wt. 44 000. Only the 38 000 mol. wt. form was detected after storage of this medium and no change in total renin activity was evident. This suggests that the 38 000 mol. wt. form may be a methodological artifact due to the release of other proteolytic enzymes.

6. With homologous substrate, all fractionated enzymes showed a pH optimum between pH 7.0 and 7.5.

7. The present study indicates that the predominant form of renin released from rat kidney into blood has mol. wt. 44 000. The microheterogeneity of the plasma enzyme with respect to isoelectric point is similar to that of kidney renin. One form of renin (mol. wt. 44 000, pI = 5.0) may be common to kidney, plasma and aorta. The second aortic enzyme (pI = 5.3), however, may be of local origin.

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