Kinetic Studies of Hypoxanthine-Guanine Phosphoribosyltransferase in Intact Cells
Since the discovery that a deficiency of the enzyme hypoxanthine guanine phosphoribosyl transferase (HPRT) was responsible for the Lesch-Nyhan syndrome (1,2) and other related ailments of greatly differing severity (3-5), enzyme assays using tissue lysates have failed to demonstrate a correlation between the degree of the enzyme deficiency and the severity of the accompanying clinical manifestations (5). Presumably this is because a mutant enzyme molecule may have decreased stability and show less activity after cell lysis than it actually possesses in vivo (4-6). Particularly noteworthy are cases in which individuals with virtually no demonstrable HPRT activity in their erythrocyte lysates show only hyperuricemia, without the behavioral and neurological abnormalities seen in the Lesch-Nyhan syndrome (5). Several investigators have shown that the activities measured in cell lysates may differ considerably from those measured in the intact cell (4,7). Using the incorporation of radiolabeled hypoxanthine into purine compounds as a measure of HPRT activity in the intact cell, it has been possible to show a close correlation between residual enzyme activity and the severity of the clinical symptoms in HPRT deficiency diseases (9). It appears that this more physiological approach is of greater value in assessing in vivo enzyme function. It is possible that cases in which there is a partial deficiency of HPRT with intermediate severity of symptoms may represent HPRT molecules with increased Km’s.
KeywordsIntact Cell Residual Enzyme Activity Erythrocyte Lysate Lanthanum Chloride Roller Bottle
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