Evidence for control of several different tissue-specific isozymes of adenosine deaminase by a single genetic locus.

Abstract

ADENOSINE deaminase is found in several different forms in human tissues. In red blood cells, adenosine deaminase activity is exhibited by a protein which has a molecular weight of approximately 35,000 (ref. 1) and is genetically polymorphic in that several inherited variants of the enzyme are distributed in normal populations (RBC ADA)2. These variants can be distinguished by differences in their electrophoretic mobility in a supporting medium of starch gel. The common form of the enzyme (ADA 1), (present in approximately 90% of a European population), is a protein which migrates more rapidly to the anode that the enzyme protein of the rarer phenotype (ADA 2). Individuals, heterozygous at this locus (ADA 2–1), possess both proteins. In addition, each of these molecules gives rise, presumably through secondary modifications of the primary gene product3, to two additional, enzymatically active proteins, with one or two more net negative changes. This results in the typical triple banded pattern of enzyme activity seen after electrophoresis of red cell haemolysates obtained from individuals homozygous at this locus (Fig. 1). Tissues other than red blood cells contain, in addition to the polymorphic enzyme, one or more additional proteins or isozymes exhibiting ADA activity1. These proteins vary in electrophoretic mobility in a manner specific for any particular tissue, but most of them exhibit a slower mobility during starch gel electrophoresis than do the RBC isozymes and have a larger molecular weight of 230–280,000 (refs 1 and 4). In addition, liver and lung tissue contain a more labile isozyme which has an electrophoretic mobility intermediate between that of the first and second bands of RBC ADA 1 and which is of undetermined molecular weight. These ‘tissue specific’ isozymes do not show the polymorphism observed in the RBC form of the enzyme, in that they do not vary in electrophoretic mobility in individuals of differing ADA RBC phenotypes. The several tissue-specific isozymes are therefore considered to be determined at one or more separate genetic loci1 not allelic with the locus for RBC ADA.