Thyroid Function and Kidney Muramidase. ∗

Abstract

Animal experiments have shown that both endogenous and exogenous muramidases accumulate in the kidney(1,2,3). Muramidase activity is not normally found in urine(4,5,6) but appears there when the blood muramidase is elevated. The increase in kidney muramidase and its appearance in urine have been interpreted to be associated with the excretory function of the kidney rather than an indication of synthesis of the enzyme by this organ(7). There is, however, no information regarding the form in which the enzyme is stored by kidney tissue.

Muramidases are basic proteins and bind with many acidic compounds in tissues to form more or less stable complexes(8,9,10). For example, muramidase combines with heparin in vitro and in vivo to form a reciprocally inactive complex(11,12). Muramidase also combines in vitro with thyroxine (13,14) to form an inactive complex. However, no confirmation of this type of inactivation has been obtained in vivo. Kidney muramidase activity in rats fed thyroid is decreased(15,16), suggesting an enhanced formation of an inactive muramidase-thyroid complex and increased storage of it in the kidney at the expense of the free enzyme. The demonstration of such a complex in vivo would be important physiologically. Biochemically it would indicate a new function of muramidase in mammalian tissue. Therefore, the possible relationship between the functional state of the thyroid and kidney accumulation of muramidase was investigated in this study.

Materials and methods. Normal and hypophysectomized rats, male and female of the Sprague-Dawley strain (Charles River) were maintained on Purina Laboratory Chow. Approximately 200 animals were used. Experiments were repeated at least twice, and typical results are presented. Egg white muramidase (EWM)† was dissolved in phosphate buffer pH 7.3 1/15 m and administered i.p.

Tuberculobacillus Calmette-Guerin (B.C.G.) (Phipp strain) from a 14-day-old culture in Dubos Medium‡ was injected i.v. into each animal as a saline suspension of 10 mg (wet weight).