Nicotinate phosphoribosyltransferase (
NAPRTase) in Escherichia coli mediates the formation of
nicotinate mononucleotide, a direct precursor of
nicotinamide adenine dinucleotide (
NAD), from
nicotinate and 5-phosphoribosyl-1-pyrophosphate. Specifically,
NAPRTase contributes to
NAD synthesis by utilizing intracellular
nicotinate formed from
NAD degradation products, which are recycled by
NAD cycle
enzymes and exogenous
nicotinate when it is available. In previous studies, it has been tacitly assumed that almost all
NAD cycle
enzymes are localized in the cytoplasm of E. coli. The results of this investigation provide evidence that
NAPRTase is a periplasmic (extracytoplasmic)
enzyme. The osmotic shock of exponential-phase cells of E. coli K-12 and ML 308-225 resulted in the release of 63 to 72% and 42 to 48%, respectively, of the
NAPRTase into the
shock medium. In addition, when exponential cells of strains K-12 and ML 308-225 were converted into spheroplasts, 75 to 84% and 54 to 68%, respectively, of the
enzyme was released into the spheroplast medium. Since previous estimates of the effective levels of
NAPRTase present in putative repressed and derepressed E. coli cells appeared to be very low, a more convenient and accurate alternative method for the evaluation of
NAPRTase in whole cells was developed. The results show that
NAPRTase is subject only to a modest degree of enzyme repression. In addition, no evidence was found for the presence of a
protein or low-molecular-weight inhibitor of the
enzyme in repressed cells.