Abstract
Initial velocity and ATP-ADP isotope exchange kinetic data on yeast nicotinate phosphoribosyltransferase indicate a sequential steady-state mechanism m which ATP adds first, followed by a random order of addition of the substrates PP-ribose-P and nicotinic acid. Reaction flux through the alternative ping-pong pathway, evidenced first by ATP-ADP isotope exchange in the absence of nicotinic acid and PP-ribose-P substrates, is reduced by low concentrations of either, or both, of these substrates. Both Mg 2+-complexed and -free PP-ribose-P are enzymatically active. Phosphorylated enzyme, prepared with [γ- 32P]ATP and separated by gel chromatography, retains 32P i until incubated with both substrates, PP-ribose-P and nicotinic acid, which completely discharge the 32P i.
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