Assay of leucine aminotransferase in rat tissues and tumors

A survey of leucine aminotransferase activities in an extensive variety of rat tissues further defined the unique distribution of this enzyme. Its reaction was measured in cell-free extracts under optimal conditions, and its product assayed both colorimetrically and with 14CO2 formed from it by decarboxylation with H2O2. The two methods agreed, though the second was more precise and dependable. In all the tissues and cell fractions examined, except adult liver, the activities with valine and isoleucine were parallel and similar to that with leucine. In adult liver there was low activity only with leucine, which is referable to a known isozyme (type II). Although the fraction of enzyme activity present in mitochondria has often been neglected, these studies showed that this portion in all tissues is about equal, more or less, to that of the more familiar soluble forms. There was good agreement between the relative concentrations of this enzyme in the commonly measured rat tissue preparations as determined here and in the more recently published studies. The unique occurrence of high levels of this enzyme in heart, kidney, brain and muscle, in that order, was confirmed. The unusual distribution of this enzyme was emphasized by the recognition that equally high or even higher levels also occurred in pancreas, lactating mammary gland and salivary gland. The relative amounts were less in normal, undifferentiated and in neoplastic tissues.