USEFULNESS OF INTACT ERYTHROCYTE STUDIES IN THE DIAGNOSIS OF INHERITED PURINE AND PYRIMIDINE DEFECTS: 62

Abstract

Diagnosis of the degree of severity of some inherited defects may be difficult where the normal enzyme is either very labile, or where the mutant enzyme is extremely unstable.Studies in intact red cells using physiological conditions and [8-¹⁴C] labelled substrates (lmM P_i:10μM),or PP-ribose-P generating conditions (18mM P_i) have proved invaluable in such cases. Under these conditions control red cells at lmM P_i convert less than 35% of adenine or hypoxanthine to the nucleotide,but more than 80% at 18mM P_i.

Studies in a variety of inherited disorders have shown: (1) that despite lack of detectable lysate activity,some partially HGPRT deficient patients have up to 25% of normal activity in intact cells; (2) raised endogenous PP-ribose-P levels are evident in the Lesch-Nyhan syndrome and PNP deficiency from the almost complete conversion of substrate even at lmM P_i, but PP-ribose-P levels are normal in ADA, APRT and OPRT/ODC deficiency. These defects may also be detected using intact red cells and the appropriate substrate; (3)that despite normal lysate APRT and HGPRT activity, cells from a PP-ribose-P synthetase mutant failed to respond to phosphate activation using either substrate; (4) the presence of an unstable enzyme in some heterozygotes for ADA deficiency,where the same red cells showed no detectable lysate activity,but up to 60% of normal conversion of deoxy-adenosine (10 μM) - at either 1 or 18mM P_i - using intact cells.

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