At least eight different carrier proteins transport different groups of amino acids.a. Sodium-amino acid carrier system(1). A number of transport systems involve the uptake by the cell of a sodium ion and an amino acid by the same carrier protein on the luminal surface (examples are systems A, ASC, N, and B).(2). The sodium ion is pumped from the cell into the blood by the Na+-K+ ATPase, whereas the amino acid travels down its concentration gradient into the blood.(a). Thus, the transport of amino acids from the intestinal lumen to the blood is driven by the hydrolysis of adenosine triphosphate (ATP) (secondary active transport).b. The L-system of amino acid transport is not dependent on sodium for cotransport. The L-system transports branched-chain and aromatic amino acids and will be exploited in a treatment for phenylketonuria (PKU).The individual has developed pellagra caused by a lack of dietary niacin. Although dietary niacin is the major source of the nicotinamide ring of NAD, it may also be produced from excess tryptophan. Tyrosine, thiamine, thymine, and riboflavin cannot contribute to the synthesis of the nicotinamide ring of NAD.