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

Renal processing of low molecular weight proteins

Previous renal clearance studies provided quantitative data concerning renal reabsorption of proteins while the simultaneous processes of renal accumulation and degradation remain, to a great extent, insufficiently investigated. Thus, it was the aim of this study to measure renal reabsorption of egg-white lysozyme at various lysozyme concentrations and to relate the corresponding accumulation and degradation of lysozyme to the lysozyme transport rates in intact rats and isolated perfused rat kidneys. Lysozyme (with 125I-lysozyme in certain experiments), was continuously infused i.v. or added to the perfusate to achieve plasma (or perfusate) concentrations of lysozyme (PLY) of approximately 50, 500 or 1000 mg X 1(-1) for periods of time varying between 3 and 120 or 150 min. Clearances of inulin and lysozyme or the total content of radioactivity and the trichloroacetic acid (TCA)-soluble radioactivity in the kidney tissue were determined at the end of clearance or accumulation periods. Additionally the perfusate concentration of the metabolite tyrosine was measured by high performance liquid chromatography (HPLC). The reabsorption rates of lysozyme (TLY) were concentration-dependent in both intact rats and isolated perfused rat kidney. After 25 min of lysozyme infusion, the lysozyme reabsorption rates amounted to 37, 245 and 331 micrograms X min-1 X g-1 kidney at the above lysozyme concentrations. After the same infusion time, the accumulation rates of lysozyme were 8, 59 and 118 micrograms X min-1 X g-1 kidney. The difference between the transport rate and accumulation rate should represent the renal degradation rate of lysozyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of aminoglycosides on glomerular permeability, tubular reabsorption, and intracellular catabolism of the cationic low-molecular-weight protein lysozyme

Gentamicin and other aminoglycoside antibiotics in high doses may produce proteinuria and other signs of nephrotoxicity. Proteinuria may result from general renal damage or may reflect alterations in specific steps in the renal handling of proteins. To distinguish between these two possibilities, experiments were designed to quantify the effects of nephrotoxic doses of several aminoglycosides on the renal handling of proteins in the isolated perfused rat kidney with the cationic low-molecular-weight protein lysozyme as a representative protein. Each aminoglycoside was administered ip to male Wistar rats (30 mg/kg/day) for 7 days. Lysozyme and 125I-lysozyme were added to the perfusate to achieve a lysozyme perfusate concentration of about 100 mg/liter. Clearances of inulin and lysozyme, release of tyrosine and trichloroacetic acid-soluble radioactive metabolites into the perfusate, and the glomerular sieving coefficient of lysozyme were determined. Scanning and transmission electron microscopy indicated that gentamicin and tobramycin decreased the number and diameter of the endothelial fenestrae of the glomerular capillaries. Concurrently, gentamicin and tobramycin decreased the glomerular sieving coefficient of lysozyme from 0.8 to 0.6 and 0.5, respectively. Netilmicin did not affect the percentage reabsorption of lysozyme whereas gentamicin and tobramycin decreased lysozyme reabsorption from 71.7 to 35.4 and 34.4% of the filtered load, respectively. Lysozyme degradation, estimated by the release of tyrosine into the perfusate during a 150-min perfusion period, was decreased from a control value of 12 mumol/liter to 4.43 and 4.65 mumol/liter in kidneys from rats treated with gentamicin and tobramycin, respectively. This study demonstrates that polycationic aminoglycosides may affect several processes involved in renal handling of lysozyme including glomerular permeability, tubular reabsorption, and intracellular proteolytic degradation.