Does angiotensin regulate systemic conversion of prorenin to renin?

Potent 'new pressor protein' related to coagulation factor XII is potentiated by inhibition of angiotensin converting enzyme

BACKGROUND

'New pressor protein' was observed after tryptic activation of human and rat plasma in vitro, which is done conventionally for prorenin measurements.

RESULTS

It is potently pressor, heat labile, possesses enzyme activity, and has a relative molecular mass > 30 kDa with isoelectric point(s) 4.7-4.9. New pressor protein equivalent to only 0.01 ml human, or rat, plasma injected intravenously quickly raises systolic blood pressure in 300 g anesthetized, ganglion-blocked, bioassay rats by about 15 mmHg. For unknown reasons, this is potentiated to about 45 mmHg after treatment with angiotensin I converting enzyme inhibitors (such as captopril and enalapril). New pressor protein activity in rats remains normal 24 h after bilateral nephrectomy, suggesting that it has an extrarenal origin and, furthermore, excluding the possibility of an association with renin-angiotensin system. Systolic blood pressure elevation is greater than the diastolic one, implicating cardiotonic effects. Human plasma new pressor protein was purified using standard biochemical techniques and its N-terminal sequence (19 residues) found to be homologous with the beta factor XIIa fragment of coagulation factor XII. This was supported by demonstrating inhibition of new pressor protein activity in vitro using the factor XII-specific corn trypsin inhibitor. Also, human new pressor protein activity in humans congenitally deficient in coagulation factor XII is very low. The high potency and multiphasic, cardiotonic effects of injected new pressor protein suggest that it interacts synergistically with other systems in the body. This was confirmed by showing that, within 10 min of total bilateral adrenalectomy, responses to new pressor protein decreased markedly.

CONCLUSIONS

New pressor protein's action requires adrenal (medullary?) involvement, but its mechanism of action and that of its potentiation by angiotensin converting enzyme inhibitors remain unknown. The physiologic and clinical relevance of these observations depends on whether activation of new pressor protein can occur in vivo.

Human renin activation by protease from the renin granule fraction of the dog kidney cortex

To clarify the possible conversion of prorenin in renin granules where conversion reportedly occurred, we investigated whether the renin granule fraction of the kidney could activate prorenin to the active form. Renin granules were isolated from the dog kidney cortex by discontinuous sucrose density gradient centrifugation. Human active renin was quantified by immunoradiometric assay which could detect only the human active renin but not the inactive human renin or dog renin. Inactive renin from human amniotic fluid was incubated with the subcellular fraction of the dog kidney cortex. The renin granule fraction that showed the highest renin activity stimulated the inactive renin to become the active form. The membrane preparation obtained from the renin granule fraction by freezing and thawing the fraction in low osmolarity retained the activity of renin activation. Other subcellular fractions showed less renin activation. The optimal pH for renin activation by the membrane was pH 5.0 to 6.0. The activation depended on the time of incubation and concentration. The activation was inhibited by N-ethylmaleimide but not by EDTA or serine protease inhibitors. These results suggest that renin is processed by a membrane bound protease in renin granules.