'Late onset' ornithine transcarbamylase deficiency: function of three purified recombinant mutant enzymes

Although many mutations in the ornithine transcarbamylase gene have beencorrelated with 'late onset' of hyperammonemia in patients, the effects ofthese mutations on enzyme function are largely unknown. Three recurrentmutations (R40H, R277W and R277Q) found in patients with 'late onset'disease were incorporated into 'mature' human ornithine transcarbamylasecDNA and overexpressed in Escherichia coli. The three recombinant mutantenzymes were purified to homogeneity on an affinity column and theirbiochemical characteristics were compared to the wild type enzyme. TheR277W and R277Q mutants display markedly reduced affinity for L-ornithine,loss of substrate inhibition, alkaline shift of pH optimum, and reducedthermal stability compared to the wild type enzyme. These differences,particularly the reduced affinity for L-ornithine, are sufficient toaccount for their biochemical effects. In contrast, the 'mature' R40Hmutant was biochemically indistinguishable from the wild type enzyme invitro.