The determination of serum vitamin B12 concentrations has been the method of choice to assess status for many years. Acceptable concentrations are 147pmol/L (200 mg/L) and a risk of deficiency is indicated by concentrations <74pmol/L (100 mg/L). Most circulating vitamin B12 is carried on TC I, which is biologically inactive, but representative of the status of the individual. Conventially TC I was measured by a microbiological assay which has been the 'gold standard'. Recently a radiometric assay based on competitive binding between endogenous vitamin B12 and an exogenous radioactive form of vitamin B12 for a binder has been used. It has been suggested that measuring holo TC II might be a better way of assessing cobalamin deficiency. However, a recently introduced indirect measurement of intra-cellular cobalamin deficiency may be a more functional measure of B12 status (Green, 1995). It is the measurement of the substrates of two cobalamin-dependent enzymes, methylmalonic acid (MMA) and homocysteine.
Normal variations in dietary intake of MMA do not affect plasma levels. MMA is excreted in the urine and is closely correlated with plasma concentrations. Plasma MMA levels rise in renal failure and in cobalamin deficiency but not in folate deficiency. Plasma concentrations rise from normal values of 0.10.4 mmol/L to 50-100 mmol/L in vitamin B12 deficiency. MMA in urine or plasma is a sensitive measure of absolute and/or functional vitamin B12 deficiency, and is especially useful in the diagnosis of sub-clinical vitamin B12 deficiency in the elderly.
Plasma homocysteine comes predominantly from dietary methionine, although concentrations are sensitive to dietary vitamin B12, folate and pyridoxine, the vitamin co-factors associated with its metabolism (Fig. 3.3). Normal homo-cysteine concentrations are higher in men than in premenopausal women, but increase with age especially after age 60.
Plasma homocysteine concentrations rise in deficiency states of folate, vitamin B12 and pyridoxine, in persons with inborn errors of the enzymes of homocys-teine metabolism and in those with defects associated with the synthesis of vitamin B12 coenzymes required for normal function of methionine synthetase. However, plasma MMA concentrations seem to be more specific and sensitive indicators of vitamin B12 status than homocysteine concentrations and are significantly better than plasma vitamin B12. It is not unknown for normal individuals with normal plasma vitamin B12 concentrations to have had elevated levels of MMA that fell after a single injection of vitamin B12. In vitamin B12 deficiency, elevated levels of both MMA and homocysteine seem to occur before a fall in plasma vitamin B12 (Green, 1995).
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