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A sensitive and precise assay of guanase was based on the conversion of 14C-guanine to uric acid in the presence of excess xanthine oxidase. The enzyme was entirely soluble in rat tissues and no inhibitors of it were detected. The most active tissues were red blood cells, lung and lactating mammary gland, with more than twice the activity of liver. The contained blood could account for the low activity in adult skeletal muscle and some other tissues. All fetal tissues examined were without activity. Activity in mammary gland rose fourfold during lactation and dropped precipitously during involution, with a secondary rise associated in time with the loss of cells from the gland. Reticulocytes present normally and after hemorrhagic anemia appeared to account for substantially all of the high guanase activity in red blood cells. The guanase level could be used to predict the degree of reticulocytosis in rats within confidence limits of +/- 0.2%. The virtual absence of guanase in human red cells was confirmed, even in bloods containing elevated reticulocyte numbers.

The behaviour of guanase activity and the 8-azaguanine/guanine deamination ratio of rat tissues during postnatal development were studied. In the liver and brain, the enzymatic activity, present from birth, gradually increases up to the 4th week. In the kidney, guanase activity appears after the 1st week and reaches the level found in adult tissues after the 6th week. The value of the deamination ratio of tissues at birth is higher than that of adult animal tissues.

The activation energy and the optimum pH of guanine deaminase in man, the rat, guinea pig and mouse were studied using 8-azaguanine as a substrate. The serum guanase in man and in all the animal species studied differs in activation energy from the guanase of the liver. In man, moreover, the serum guanase is also different from the brain and kidney enzyme. In the rat and guinea pig the brain enzyme has thermic activation energy different from the liver and kidney enzyme. The guanase of the serum and tissues of the guinea pig differs from the enzyme of the serum and tissues of man, rat and mouse for optimum pH.

Some properties of serum and tissue guanase from man, rat, mouse and guinea pig have been studied. Serum guanase differs from tissue guanase in activation energy in the human species and in guinea pig. The mouse and rat brain enzyme differs from the liver and kidney enzyme. The guanase of each species differs from the enzyme of the other species in one or more properties.

Serum guanase activity was measured in 20 healthy adults and in 62 patients with acute viral hepatitis, chronic active hepatitis, chronic persistent hepatitis, liver cirrhosis and fatty liver. Guanase and gamma-GT in patients were elevated in 87 and 64%, respectively. Elevated guanase activities were found in most cases of acute viral hepatitis, as well as in chronic hepatopathies. In patients with acute viral hepatitis pathologic activities of guanase were found following partial or total normalization of other liver function tests.