Spontaneous inhibition or inactivation of renin in rat plasma.

Renin appears to be rapidly inactivated in vitro. The present study was undertaken to clarify this observation and to establish the existence of substances involved in renin inactivation. The disappearance rate of renin (including pure renin) was measured in plasma incubated at 37 degrees C and in circulating blood. Pure renin added to plasmas disappears in vitro at the same rate (t1/2 approximately or equal to 40 min) that renin in plasma from normal rats and from rats submitted to a hemorrhage. This process appears not to be mediated by proteases. The disappearance rate of endogenous renin in the normal group (n = 18) was 39.7 min with rapid phase (R) of t1/2 = 14.2 min and a slow phase (S) of t1/2 = 94.3 min), whereas it was 32.1 min (t1/2 R = 13.1 min and t1/2 S = 69.1 min) in rats submitted to an hemorrhage (n = 6). The t1/2 of pure renin was 31.4 min (t1/2 R = 13.3 min and t1/2 S = 69.2 min). Incubation of plasma reveals that renin is inactivated or inhibited in vitro at a similar rate than in circulating plasma. These results suggest that inactivation and sequestration of renin could be two independent mechanisms in the maintenance of plasma renin activity.

Activation of renal renin by a protein plasma fraction: a novel enzymatic mechanism.

A 3-fold increase in active renin was found after a kidney cortex extract was incubated with plasma from either normal or nephrectomized rats (0.34 +/- 0.04 to 1.34 +/- 0.08 and 1.60 +/- 0.06 micrograms Angiotensin I/mg tissue/hr, respectively). A plasma protein that activates renal renin was purified 900-fold. Purification of the protein was achieved by a combination of ammonium sulfate fractionation, molecular filtration on Sephacryl S-200 HR and ion-exchange chromatography on Mono Q HR 5/5 associated to an fast performance liquid chromatography (FPLC) system. The protein shows a molecular weight of approximately 54,000 Da. Renin activation was not inhibited by serine protease inhibitors, such as phenylmethyl sulfonylfluoride, aprotinin, soybean trypsin inhibitor and N-tosyl-L-phenylalanine chloromethyl ketone or by the cystein protease inhibitors N-ethylmaleimide and leupeptin. By using enzyme inhibitors, it was found that the activation process is not mediated by kallikrein, plasmin, tonin, cathepsin B or trypsin-like enzymes. From these results, we conclude that there is in circulating plasma a previously unidentified enzyme capable of activating inactive kidney renin. However, the possibility that this protein acts by activating the renin-substrate reaction cannot be dismissed.

Multiple forms of acetylcholinesterase from rat erythrocytes. Effect of fat-free diet.

Electrophoretic patterns of acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) from rat erythrocyte were studied. The enzyme was solubilized by the following treatments: a) Triton X-100, b) sodium deoxycholate, or c) ultrasonic irradiation. When the erythrocyte membrane was solubilized by Triton X-100 at concentrations higher than 0.3%, by 10 mM sodium deoxycholate, or by ultrasonic irradiation for more than 5 min, a single band of acetylcholinesterase activity appeared in the gel. Two bands of activity were stained in the gel when the membrane was solubilized by Triton X-100 at concentrations between 0.1--0.2%, or by ultrasound for 5 min. Electrophoretic patterns of acetylcholinesterase from rats fed a fat-sufficient diet were similar to those for the enzyme from animals fed a fat-free diet. The recombination of lipids with the enzyme eluted from the gels confirmed the "phenotypic allosteric desensitization phenomenon".