Isolated perfused intestine of the rat will be used to study the digestion, absorption and metabolism of the multiple precursors of pyridine nucleotides in pyridoxal phosphate. Tryptophan, quinolinate, nicotinate, or nicotinamide labeled with carbon-14 and pyridoxine, pyridoxal and pyridoxamine and their 5' phosphates labeled with tritium, will be infused vascularly and luminally, to determine the reactions of digestion and metabolism which each undergoes in the intestine in preparation for transport to and utilization by other organs and tissues. The intestine, its contents and the perfusate will be examined in a kinetic study of interconversions. When the dominant reactions are identified, the responsible enzymes will be examined.