Studies on the reaction mechanism of a ribonuclease II from Aspergillus oryzae (author's transl)]

Ribonuclease T2 was isolated from an Aspergillus oryzae extract. In order to define the substrate specificity, the hydrolysis of a series of 2',3'-cyclic nucleotides was measured semiquantitatively. Modifications in all positions of the bases are tolerated, as long as the base stays in the anti conformation or has a chance to return to it; bulky substituents at N-3 of the pyrimidine base lower the rate. So far the conclusion seems justified that the enzyme does not react with the substrates by specific bonds to the base, but rather by hydrophobic binding. The conformation specificity and the pH dependence of the activity support this hypothesis. The pH optima with substrates which may be positively or negatively charged are shifted to pH values at which the substrates are uncharged. This strongly indicates a hydrophobic type of interaction between base and enzyme. From the pH dependence of the kinetic parameters Km and k+2, an enzyme group with a pK of 7 (probably histidine) can be postulated. This group should interact in the protonated form with the phosphate anion. Another B.HB-system (probably two carboxylate groups) seems to be involved in the catalysis step, performing the base catalysis at the 2'-OH group and the proton catalysis at the phosphate oxygen simultaneously.     

Releasing of plasminogen-activator from the kidney to the blood (author's transl)]

When fibrinolytic activity in blood samples from various vessels was examined by the dilute-blood-clotlysis-time method (DBCLT), it was found to be noticeably high in the renal venous blood, though the activity was not detected by usual blood clotlysis time method. Plasmin was not detected in any blood samples examined, and the contents of fibrinogen and fibrin (or fibrinogen) breakdown products in the renal venous blood were not significantly different from those in the blood from other vessels. However, the high activity of plasminogen-activator was found only in the renal venous blood. Inhibitors on plasmin and plasminogen-activator (urokinase) were detected in almost the same amount in the blood samples from the various vessels. The amount of the inhibitors was sufficient to inhibit the plasminogen activation by urokinase, whose activity was equivalent to the plasminogen-activator activity in the renal venous blood. These results indicate that the high activity by DBCLT in the renal venous blood was derived from the high activity of plasminogen-activator, which was inactivated by inhibitors in undiluted blood. Plasminogen-activator may be released from the kidney to the blood, and immediately inactivated by the inhibitors in renal vein, and then diluted with systemic blood which contains little plasminogen-activator.