We have investigated the molecular basis of hypoxanthine-guanine 
phosphoribosyltransferase (HPRT) deficiency in a patient who presented with the 
Lesch-Nyhan syndrome. A catalytically incompetent form of HPRT has been isolated 
from this patient's erythrocytes and lymphoblasts. This enzyme variant, which we 
have termed HPRTKinston, is indistinguishable from the normal enzyme in terms of 
its intracellular concentration and maximal velocity, but differs with respect 
to its isoelectric point (more basic) and Michaelis constants for both 
substrates (markedly elevated). The tryptic peptides of HPRTKinston were mapped 
by reverse-phase high pressure liquid chromatography in an attempt to define the 
precise abnormality in its primary structure. Sequence analysis of the single 
aberrant tryptic peptide in HPRTKinston revealed an aspartic acid to asparagine 
amino acid substitution at position 193. Electrophoretic analysis of the CNBr 
peptides of HPRTKinston confirmed the location of the proposed mutation. This 
amino acid substitution can be explained by a single nucleotide change in the 
codon for aspartic acid 193 (GAC leads to AAC). This is the first specific 
mutation described at the molecular level in a patient with the Lesch-Nyhan 
syndrome.