A covalent adduct of an aminoacyl tRNA synthetase and uracil nucleoside has been isolated. The enzyme adduct is catalytically inactive; one nucleoside is bound per catalytic site. The release of uridine restores enzyme activity. The nucleoside attaches to a protein segment required for tRNA interaction. The findings add support to concepts of a covalent component for some protein-nucleic acid complexes.

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Dissociation constants have been measured for the binding of a variety of simple analogues of pyridoxal 5'-phosphate to apoglutamate decarboxylase. Compounds studied have a simple alkyl or aryl group and a negatively charged substituent (phosphate, phosphonate, phosphoramidate, sulfate, sulfonate, or carboxylate). Optimum binding to the phosphate binding site of the enzyme is achieved by compounds having a double negative charge and a tetrahedral geometry. Planar anions and monoanions bind considerably less well. These and previous data are used to to derive the magnitudes of the contributions of various coenzyme functional groups to the strength of the apoenzyme-coenzyme interaction.

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