Use of gelatin-coated charcoal in digoxin immunoassay.

(1) showed that pyridoxal-5'-phosphate (a phos-phorylated derivative of vitamin B6) is essential for serum aspartate trans-aminase (AST) activity.This coen-zyme is bound to an amino group of the enzyme during transfer,becoming pyridoxamine phosphate (2). In 1954, Karmen et al. (3) described a practical method of measuring AST activity. Although they gave no data, they did add buffered pyridoxal phosphate to the reaction mixture and found it had no significant effect. Amador and Wacker (4), in their analytical assessment of current assay techniques, make no mention of pyridoxal phosphate and apparently assume that it is present in adequate amounts. Producers of commercial AST assay kits assume the same. We show here that added pyridoxal phosphate significantly affectsAST activityof serum, leading in some cases to falselylow values ifitisnot added.