Methylmalonyl-CoA mutase activity of leukocytes in variants and heterozygotes of methylmalonic acidemia

Assay of methylmalonyl CoA mutase with high-performance liquid chromatography

An assay for methylmalonyl CoA mutase activity is described. Succinyl CoA produced in this method is separated from the substrate, methylmalonyl CoA, by reverse-phase high-performance liquid chromatography and is quantified. This method is useful to differentiate mutase apoenzyme deficiency (mut0, mut-) and the defect in deoxyadenosylcobalamin synthesis using fibroblasts cultured in high concentration of supplementary hydroxocobalamin. In methylmalonic acidemia, measurement of lymphocytes mutase activity offers therapeutical and prognostic informations.

Dietary therapy in two patients with vitamin B12-unresponsive methylmalonic acidemia

The biochemical and therapeutic responses to dietary therapy were studied in a 25-month-old girl and a 1-month-old girl with methylmalonic acidemia (MMA-emia), which was unresponsive to vitamin B12. The minimum daily intake of protein which patients could tolerate and display a good development was between 1.0 and 1.2 g per kg body weight. Supplementation with amino acid mixture devoid of toxic amino acids was required to prevent protein malnutrition when daily protein intake was restricted to 0.6 g per kg body weight. Caloric intake should be sufficient, not only to promote growth but also to prevent a rise in MMA level, especially when a patient has ketoacidosis. It was found that MMA excretion per mg creatinine in random urine specimens correlated significantly with serum MMA and twenty four-hour output of MMA per kg body weight. Therefore measurement of MMA in a single urine specimen is useful for evaluating the in vivo accumulation of MMA.

Inherited methylmalonyl CoA mutase apoenzyme deficiency in human fibroblasts: evidence for allelic heterogeneity, genetic compounds, and codominant expression

We have measured and characterized methylmalonyl coenzyme A (CoA) mutase activity in extracts of cultured human fibroblasts from 23 patients with inherited deficiency of the mutase apoenzyme and from eight obligate heterozygotes for this defect. The mutant cell lines fall into two categories. Those without detectable residual mutase activity in cell extracts (>0.1% of control), and whose ability to utilize propionate in intact cells is refractory to supplementation of the culture medium with hydroxocobalamin, are designated mut degrees mutants. Those with detectable residual activity in cell extracts ( approximately 0.5-50% of control), and whose ability to utilize propionate in intact cells in markedly increased by hydroxocobalamin supplementation, are designated mut(-) mutants. The mutant enzyme in the mut(-) mutants exhibits a 50- to 5,000-fold elevated Michaelis constant (K(m)) for adenosylcobalamin in vitro, a normal K(m) for methylmalonyl CoA, and a strikingly reduced thermal stability at 45 degrees C relative to control. Mutase from one mut(-) mutant turns over at a rate three to four times that of control enzyme when cells are grown in hydroxocobalamin-supplemented medium.To detect heterozygous carriers of mutant mut alleles, we compared mutase activity in fibroblast extracts from four controls with that from eight parents of either mut degrees or mut(-) mutants. After cell growth in either unsupplemented or hydroxocobalamin-supplemented medium, activity in cell lines from heterozygotes was reduced to 47 or 37% of the mean control activities, respectively. We also examined the effect of adenosylcobalamin concentration on reaction kinetics of mutase from heterozygote cell lines. All four cell lines from parents of mut(-) mutants exhibited complex enzyme kinetics; approximately 80% of mutase activity demonstrated a K(m) indistinguishable from control, whereas a smaller component of activity exhibited a K(m) similar to the abnormal K(m) expressed by the mut(-) propositus in each family. In two families with a mut degrees propositus, mutase from three of the four parents exhibited only the normal K(m) for adenosylcobalamin, whereas mutase from one parent displayed complex kinetics, indicating expression of both a normal allele (mut(+)) and a mutant allele with an abnormal K(m). From these studies, we conclude that mut mutants reflect mutations at the autosomal gene locus for the methylmalonyl CoA mutase apoenzyme; that mut degrees , mut(-), and mut(+) alleles at this locus are codominantly expressed; and that some mut mutants may be genetic compounds, inheriting two different mut degrees or mut(-) alleles from their parents.