Evidence for a Linear Correlation between the Level of Dietary Tryptophan and Hepatic NAD Concentration and for a Systematic Variation in Tissue NAD Concentration in the Mouse and the Rat

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Abstract
The purpose of the study was to determine the relationship of tryptophan intake (substrate availability) and tryptophan oxygenase (TO) activity to formation of nicotinamide adenine dinucleotide (NAD). Mice, both normal and adrenalectomized, and young rats were maintained on a 6 AM to 6 PM light, 6 PM to 6 AM dark schedule and fed ad libitum a synthetic amino acid diet containing various amounts of tryptophan, from zero to about three times the normal requirement for growth. Tissue free tryptophan, TO activity, and tissue NAD were measured. The animals displayed a positive linear correlation between level of dietary tryptophan and hepatic NAD concentration which was most evident during the dark period. There was some indication of a direct relationship between NAD values and tryptophan oxygenase activity at night. These results suggest that increases in enzyme activity may be necessary at the time of feeding in order to achieve the elevated levels of NAD occasioned by intakes of dietary tryptophan which are in excess of that needed for growth. Additional findings indicate the existence of a systematic variation in liver, heart and spleen NAD concentration and in hepatic NAD/NADH ratio.