Nicotinamide uptake and metabolism by chick intestine

Intestinal uptake and metabolism of nicotinamide (NAm) were studied in isolated epithelial cells and in isolated segments in situ at a physiological concentration of [14C]NAm (11.7 microM). [14C]NAm was rapidly taken up from the bathing medium and largely metabolized to [14C]NAD by the isolated cells. Total accumulation of 14C label was energy dependent and saturable at higher concentrations of NAm (148 and 351 microM). In contrast, the tissue content of NAm was unaffected by metabolic inhibitors, and the bathing media NAm rapidly equilibrated with intracellular space at all levels of NAm. NAm was converted directly to NAD via the intermediate nicotinamide mononucleotide (NMN); nicotinic acid was not an intermediate in this conversion and was a less efficient precursor of NAD. NAm absorption in vivo was substantial, 30.6% of dose after 10 min. Also, data on NAm entry into the mucosa and subsequent metabolism in vivo supported the in vitro observations. Exogenous NMN reduced NAm entry into the mucosal cells both in vivo and in vitro; the effect was specific to NAm. This is the only suggestion to date that NAm entry might proceed by some form of specialized transport process.