A Clinical Laboratory GLC Method for Urine Methylmalonic Acid

Publisher Website
Google Scholar
Abstract
A rapid sensitive GLC method for measuring urinary methylmalonic acid (MMA) employing boron trifluoride methylation of the acid is described. The method is sensitive to 1 mg MMA/L and affords MMA recoveries of 96-77 % over a concentration range of 5-2,000 mg MMA/L of urine. Mean daily MMA excretion in 10 normal subjects was 1.33 mg with a range of .56 to 3.2 mg. The daily excretion range of MMA in 5 vitamin B12 subjects was 12.5-90 mg/day. These values compare with previously published levels determined using equally sensitive procedures. The diagnostic significance of excessive urinary methylmalonic acid (MMA) excretion accompanying human vitamin B12 deficiency1-2 and the recently described inborn error methylmalonic acid uria is well established.3 Previously described methods for its measurement have employed GLC of the di-methyl ester of MMA,1,4 or its free acid.5 Less sensitive methods employing colorimetry based on the alkaline diazo chromogen of MMA6, as well as paper2 and thin layer chromatographic methods have also been described. The GLC procedure described by Cox & White1,4 employing diazomethane for methylation of MMA is the most sensitive procedure, however, it is not suitable for clinical laboratory use. The colorimetric method, though somewhat simpler, lacks sensitivity; thus in instances of vitamin B12 lack accompanied by only slight increases in MMA excretion a metabolic challenge with MMA precursors, such as valine, must be employed to elevate urinary MMA to detectable levels.7 The purpose of this communication is to describe a GLC method for determining urinary MMA employing methylation with boron trifluoride methanol8 which is both comparable in sensitivity to other GLC methods and applicable to the clinic al laboratory.