Strongly bound nucleotide derivatives in lactate dehydrogenase from the sixgill shark

Chemical analyses on lactate dehydrogenases from diverse sources reveal the presence of substoichiometric quantities of strongly bound phosphorus, pentose, and nicotinamide. The quantities of these apparent coenzyme residuals vary among species, with a relatively high incidence in the flank muscle lactate dehydrogenase of the sixgill shark. The pentose is completely released by drastic procedures such as acid precipitation or heat denaturation of the enzyme. However, it cannot be extracted by charcoal treatment of the enzyme or exchanged with reduced 3-acetylpyridine-adenine dinucleotide. The tenacity of binding suggests that the attachment of the fragments to the enzyme is covalent and that they are somehow involved in the stabilization of the enzyme.