Abnormal ornithine carbamoyltransferase in mice having the sparse-fur mutation.

Abstract

Mice with the X-chromosomal sparse-fur (spf) mutation frequently have urinary bladder stones composed mostly of orotic acid, which was identified by the following criteria: UV and IR absorption spectra, chromatographic behavior, melting point and reactivity in a specific color test. This clue led to the discovery that spf-bearing mice have an abnormal form of liver ornithine carbamoyltransferase (carbamoylphosphate:L-ornithine carbamoyltransferase, EC 2.1.3.3). Normal ornithine carbamoyltransferase has maximum activity at pH 7.6-8.0 and 80% of maximum activity at pH 10.0. The enzyme from spf males has 22% of normal specific activity at pH 7.6 but has almost twice the normal value at pH 10.0. The activities of normal and mutant enzymes as functions of the concentration of L-ornithine are also distinctly different. The apparent net deficiency of ornithine carbamoyltransferase activity in sparse-fur males is about 90%, which may account for the accumulation of orotic acid and other pathological traits of sparse-fur mice. No dissociation of the sparse-fur and abnormal ornithine carbamoyltransferase phenotypes was observed, and it is likely that the spf locus determines the structure of liver ornithine carbamoyltransferase. Mixtures of normal and abnormal activities are found in the livers of heterozygous females. The proportion of abnormal enzyme has a large variance, indicating that the gene is subject to single-active-X control, but an explicit demonstration of single-allele-expression in individual cells was not made. Since mice having the spf mutation on certain genetic backgrounds have greatly reduced fitness, sparse-fur mice may provide information about alle-viating the consequences of ornithine carbamoyltransferase deficiency in humans. Heterozygous female mice should be useful in developing reliable methods for identifying heterozygous human females and in determining if spontaneous and induced hepatomas in mice are monoclonal or multiclonal.