Transglutaminase-catalysed Formation of Coenzymatically Active NAD+ Analog • Casein Conjugates

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An NAD+ analog, N6-[(6-aminohexyl)carbamoylmethyl]-NAD+, was immobilized on bovine caseins by the action of transglutaminase. Binding of NAD+ was not observed. It appears that NAD+ analog binds with αs1- and β-caseins through formation of the γ-glutamylamine bond between the amino groups attached to the hexyl chain in NAD+ analog and the glutaminyl residues in caseins. The NAD+ analog immobilized on the caseins was enzymatically reducible by alcohol dehydrogenase; the numbers of Coenzymatically active NAD+ analog molecules were 0.4, 0.2, 0.9, and 0.3 per mol of αs1-, dephosphorylated αs1-, acetylated αs1-, and β-caseins, respectively. The number of immobilized NAD+ analog molecules was always larger than that of the enzymatically reducible one, indicating that some of the immobilized NAD+ was inactive. Michaelis constants of immobilized NAD+ analog in alcohol dehydrogenase reaction were similar to those of free forms of NAD+ and NAD+ analog. Maximum velocities were reduced to 15-30% of the free NAD+ analog. Immobilized NAD+ analog was much more stable at alkaline pH than free NAD+ and its analog. The coenzyme • casein conjugates were recovered almost completely in casein precipitated by calcium.
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