The Substrate Specificity of Pigeon Liver NAD Kinase

The specificity of pigeon liver NAD kinase for both substrates and for divalent cations has been investigated. 2. The enzyme is activated by Mg++, Co++, Mn++, and Zn++ only: the ATP‐metal complexes have different Km, values but the maximum velocities of the reactions are the same. 3. All nucleoside triphosphates investigated will act as substrates, although the Km values and maximum velocities differ. 4. The only NAD analogue found to act as a substrate was 3‐acetyl pyridine‐adenine dinucleotide (APAD). 5. The enzyme is inhibited by free Mg and free ATP. 6. The independence of substrate binding sites confirms that a random‐addition, rapid equilibrium mechanism applies to NAD kinase.