A specific radioimmunoassay for 5'-deoxyadenosyl cobalamin in serum

A specific radioimmunoassay for 5'-deoxyadenosyl cobalamin (ado-Cbl) has been developed using an antiserum raised to this cobalamin (Cbl). At a 1:100 or greater dilution the antiserum did not bind radiolabelled methyl-, hydroxo-, sulphito- or cyano-Cbl and these Cbl analogues did not compete in the radioimmunoassay even at 100-fold higher concentration. The serum concentration (range and mean +/- SD) of ado-Cbl in 30 normal subjects was 47-134 and 81 +/- 16 pg/ml. The corresponding values for total Cbl in these sera were 189-610 and 355 +/- 144 pg/ml, and the computed values for methyl-Cbl were 142-476 and 274 +/- 127 pg/ml. The coefficient of variation was substantially greater for methyl-Cbl than for ado-Cbl (46% v. 21%, respectively). The ado-Cbl concentration was in the normal range (75-95 pg/ml) in five healthy subjects with a low normal concentration (189-217 pg/ml) of total Cbl. In two subjects with low total Cbl (118 and 170 pg/ml) and without any clinical evidence of Cbl deficiency, ado-Cbl was normal (63 and 95 pg/ml, respectively). Thus, in this group, low methyl-Cbl accounted for the lower total Cbl. The concentration (mean +/- SD) of serum ado-Cbl and methyl-Cbl in six patients with low total Cbl (57 +/- 25 pg/ml) and clinical evidence for Cbl deficiency was 35 +/- 12 pg/ml and 22 +/- 22 pg/ml, respectively. This difference from the normal mean for each cofactor was highly significant (P less than 0.001). The decrease in the concentration of methyl-Cbl in Cbl deficiency was relatively greater than the decrease in ado-Cbl (92% v. 57%, respectively) which raised the relative concentration of ado-Cbl in Cbl deficiency to 61% of the total Cbl. Although a low serum methyl-Cbl is a sensitive indicator of Cbl deficiency, it may not be as specific as a decrease in serum ado-Cbl. Factor(s) other than tissue stores of Cbl may lower serum methyl-Cbl below the 95% confidence limit in the absence of clinical Cbl deficiency.