Digestion and Absorption of NAD by the Small Intestine of the Rat

Abstract

A number of preparations of varying complexity have been used in an effort to elucidate the reactions by which NAD is hydrolyzed to nicotinamide during intestinal digestion. NAD labeled with 14C in the adenine or pyridine moiety was the substrate used with perfused rat intestine, live rats, perfused live rats, with collection of portal flow, intestinal contents, mucosal tissue, or pancreatic juice. The conclusions reached are that a pyrophosphatase present in the intestinal juice and to a much lesser extent in the pancreatic juice releases 5'-AMP and nicotinamide ribonucleotide. The 5'-AMP was rapidly converted to adenosine then to inosine by bacteria-free intestinal contents. Perfused or intact intestine rapidly hydrolyzed NMN to nicotinamide riboside, which accumulated, but was not absorbed. It was slowly cleaved by an enzyme associated with the mucosal cells to nicotinamide, which was the major if not the only labeled compound absorbed.

Comparison of Metabolic Fates of Nicotinamide, NAD⁺ and NADH Administered Orally and Intraperitoneally; Characterization of Oral NADH

“…Gross and Henderson (1) revealed that NAD ϩ is efficiently digested in the small intestinal tract, producing Nam that is transported into the blood and distributed to various tissues. Nam in the circulation appears to take two metabolic pathways depending on the cellular conditions; Nam is reused for biosynthesis of NAD ϩ and NADH or it gives rise to some downstream metabolites whose physiological functions are not known.…”

“…Excess Nam is converted into nicotinamide N -oxide (Nam N -oxide), N 1 -methylnicotinamide (MNA), N 1 -methyl-2-pyridone-5-carboxamide (2-Py), and N 1 -methyl-4-pyridone-3-carboxamide (4-Py) and these catabolites are excreted into urine in mice ( 3 ). It has been shown that orally taken NAD ϩ also supplies Nam, since NAD ϩ is metabolized to Nam in the small intestinal tract ( 1 ).…”

NAD+ Metabolism and the Control of Energy Homeostasis: A Balancing Act between Mitochondria and the Nucleus

“…Interestingly, niacin is found in maize but is not bioavailable unless given an alkali treatment, a process used in Aztec and Mesoamerican times termed nixtamalization (Gwirtz and Garcia-Casal, 2014). In animal products, and probably in all uncooked foods, the NAD + and NADP + cellular content accounts for much of their dietary niacin content (Gross and Henderson, 1983), yet, as exemplified above with corn nixtamalization, their bioavailability might be affected by food processing or cooking.…”

Thiamin and Niacin in Ruminant Nutrition

“…The latter was probably formed by the action of bacterial or mammalian nucleotidases (Pike & Brown, 1975). In rats formation of nicotinamide from brush-border-associated dinucleotides has also been reported (Gross & Henderson, 1983). Thus, it appears that the ruminant host has access to both nicotinic acid and nicotinamide to meet the specific requirements of individual organs (Giesecke, 1983).…”