Renal handling of endogenous lysozyme in the rat

Quantitative studies of endogenous lysozyme (low molecular weight protein) were performed in rats. Urine and plasma concentrations of lysozyme and inulin were measured spectrophotometrically. An improved lysozyme assay (standard curve established by using egg white-lysozyme) enabled us to determine the mean plasma concentration of endogenous lysozyme (4.4 micrograms X ml-1) and the urinary concentrations of endogenous lysozyme (between 0.1 and 3.8 micrograms X ml-1. The urinary concentrations of endogenous lysozyme were found to be dependent on urinary flow rate. High urinary concentrations (ULy) were found at low urinary flow rates (V). The excreted amount of endogenous lysozyme (ULy X V) was independent of urinary flow rate and yielded a constant value of 0.02 micrograms X min-1. Mean glomerular filtration rate (GFR) was 1.2 ml X min-1 while clearance of endogenous lysozyme averaged 0.0039 ml X min-1. Inhibition of endogenous lysozyme reabsorption by cytochrome c was used to estimate the glomerular sieving coefficient of endogenous lysozyme in clearance experiments. CLy/GFR increased from a mean value of 0.0053 in control rats to 0.8 at maximal inhibition of tubular reabsorption of endogenous lysozyme by cytochrome c. Knowing the glomerular sieving coefficient, GFR and the lysozyme concentrations in plasma and urine samples, the filtered, excreted and reabsorbed lysozyme amounts could be calculated: 0.5% excreted and 99.5% reabsorbed. Reabsorbed endogenous lysozyme is stored in the kidney in high amounts (1,983 micrograms X g-1 kidney).