Role of the Kidney in Accumulation of Egg White Muramidase in Experimental Animals.∗

Muramidase† activity is highest in the kidney and the lung of normal animals(1,2); however, lung muramidase activity is not significantly changed under experimental conditions which evoke marked increases of kidney muramidase(3,4,5). The reason for this increase in muramidase activity is not known. De novo biosynthesis by the kidney, or accumulation of the enzyme from the circulatory system could be possible explanations. Evidence is presented here to demonstrate that the kidney is capable of accumulating from the blood stream large amounts of either endogenously produced or exogenously administered muramidase. Experimental. Animals used in these experiments, Sprague-Dawley rats (Charles River) and guinea pigs (Rockland Farms) were maintained on standard laboratory diet (Purina Laboratory Chow) with free access to water. Animals were sacrificed under nembutal narcosis, and organs were rapidly excised, blotted, weighed and homogenized as previously described(3,6) to prepare extracts for muramidase determinations. Blood plasma or urine was tested after dilution with the proper buffer. The enzymatic assay for muramidase activity, a modification of Litwack's method(7) has been previously described(8). Enzyme activity was calculated from a standard curve obtained with egg white muramidase‡ (EWM) and results expressed as mg equivalents of EWM per gram of wet tissue. The purity of the injected muramidase was checked by chromatography on 1RC50 resin at pH 6.90 with 0.2 m phosphate buffer. The UV absorption of the effluent was directly recorded at 280 μm using continuous flow cells§ of 0.3 ml volume and 1 cm light path in a Beckman DB spectrophotometer against buffer in the reference cell. Exp. 1. Fifty mg of EWM dissolved in 0.2 m phosphate buffer pH 7.2 were injected daily i.p. in a group of 10 rats (average body weight 135 g). In a second group the dosage was doubled.