A rapid sensitive spectrophotometric assay for the measurement of methylmalonic and propionic acids in urine is described. The assay is based upon the quantitation of propionic acid using acetyl coenzyme A synthetase isolated from baker's yeast. This enzyme is highly specific for acetate and propionate, and acetate interference is eliminated by conversion to citrate. Methylmalonic acid was assayed by converting it to propionate by heat decarboxylation and then measuring the propionate increment over the endogenous amount in the noncarboxylated sample. Studies of urine obtained from normal subjects (by isolation, partial purification, and then assay by the isotope dilution technique) demonstrated urinary excretion of less than 1 mg of propionic acid and 1–5 mg of methylmalonic acid per day. In 22 consecutive patients with documented vitamin B12 deficiency, methylmalonic acid excretion in excess of 30 mg/24 hr was found. In four other patients, with only neurologic involvement methylmalonic aciduria aided in identifying B12 deficiency as an etiologic factor. Methylmalonic acid excretion was measured by direct assay of an aliquot of urine, requiring neither a valine load nor special extraction procedures. Propionic aciduria was variably increased in B12 deficiency and did not correlate either with the severity of the deficit or degree of methylmalonic aciduria. The assay was performed on urine, but it is potentially applicable to tissue extracts. In addition, this assay method can be utilized for the quantification of urine acetate levels as well.

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