Abstract
Four NAD biosynthetic pathways have been known ; (I), NiANH_2→NMN→NAD pathway (II), NiA→NaMN→NaAD→NAD pathway (III), NiANH_2→NiA→NaMN→NaAD→NAD pathway (IV), QA→NaMN→NaAD→NAD path-way. So, the functional pathway in each organ of rats was investigated. It was found that all the four 'NAD biosynthetic pathways exist in the liver. Among the four pathways, (IV) is presumed to be the functional pathway as judged by the results that the levels of NAD and niacin in liver increased with the content of dietary Trp and that exogenous QA had a 1/9 niacin activity of NiA. In kidney, (I), (II), and (IV) were found to exist. In the three pathways, (I) was supposed to be the functional pathway as judged by the results that NiANH_2 was the main precursor supplied and that the activity of kidney aminocarboxy-muconate-semialdehyde decarboxylase was much higher than the liver enzyme activity. In heart, (I) and (II) were found but (I) was seemed to be the functional pathway as judged by the result that NiANH_2 was a main precursor sup-plied. In other organs, e,g., the small intestine, spleen, lung, pancreas, brain and muscle, only (I) was found. Thus, the pathway found in all the organs tested was (I). Although the reaction of NAD→NiANH_2 was found to occur in all organs, the reaction of NiANH_2→MNA→2-Py was found to occur mainly in liver. In the view of complexicity of the metabolic pathways of NAD and existence of the questionable bioavailability of the bound forms of nicotinic acid in foods, a straight forward method to determine blood NAD for assessing the nutritional status of niacin was established. Using this method, it was proved that blood NAD level was useful for assessing the nutritional status of niacin. NAD level of healthy human blood was about 30 nmol/ml.
