Studies on biotransformation of lysozyme. III. Comparative studies on biotransformation of exogenous and endogenous lysozyme in rats.

Exogenous hen lysozyme or endogenous rat lysozyme labeled with 131I was intravenously injected to rats with the same dosage, respectively, and the uptake and degradation of injected 131I-labeled rat lysozyme in liver and kidney were studied in comparison with those of 131I-labeled hen lysozyme. 1. Although the serum levels of both enzymes injected were almost indentical during the first 6 h, the liver uptake of 131I-labeled hen lysozyme was 2.2-fold more than that of 131I-labeled rat lysozyme at the peak time of 5 min after injection. The uptake and clearance of 131I-labeled rat lysozyme in the kidney were exclusively slow as compared with those of 131I-labeled hen lysozyme. 2. The intracellular distribution in the liver and kidney were examined by the differential centrifugation after injection of each lysozyme. The protein-bound radioactivity of each subcellular fraction was found to be the highest in the 12 000 X g (10 min) fraction in the liver and the 19 600 X g (20 min) fraction in the kidney. The relative specific activity of 12 000 X g fraction of the liver after injection increased with the time lapse. On the other hand, the relative specific activity of 105 000 X g (1 h) fraction of the liver attained a maximum within 5 min after injection and thereafter decreased. It was assumed that the mechanism of the uptake of injected 131I-labeled rat lysozyme in the liver and kidney was similar to that of 131I-labeled hen lysozyme. 3. The degradation of exogenous or endogenous lysozyme in subcellular particles was examined. From the effect of pH, activator and inhibitor on the degradation, the proteolytic enzyme to degrade the injected 131I-labeled hen lysozyme was indicated to be mainly cathepsin BL, with the optimal pH of about 5.0, and the injected 131I-labeled rate lysozyme was mainly degraded by cathepsin D, with the optimal pH of about 3.5 The in vitro degradation of exogenous and endogenous lysozymes showed a tendency similar to the in vivo clearance from the liver and kidney.