L-alanyl-L-cystine is a dipeptide composed of alanine and L-cystine, and its excretion and metabolic pathways in the human body are as follows:
I. Metabolic Pathways
Intestinal Absorption: L-alanyl-L-cystine is absorbed into the human body in the intestine. It can be taken up into the cells through peptide transporters on the intestinal mucosal cells, and then hydrolyzed into alanine and L-cystine within the cells before entering the bloodstream.
Hepatic Metabolism: L-alanyl-L-cystine that enters the bloodstream or its hydrolyzed products, alanine and L-cystine, will be transported to the liver. In the liver, alanine can generate pyruvate through transamination. Pyruvate can further participate in the gluconeogenesis pathway to produce glucose, or enter the tricarboxylic acid cycle to be completely oxidized and decomposed into carbon dioxide and water, releasing energy simultaneously. L-cystine can be reduced to cysteine under the action of cystinase. Cysteine is a precursor for synthesizing important biomolecules such as glutathione and taurine, and it can also participate in protein synthesis. In addition, it can generate substances such as sulfate through a series of metabolic reactions.
II. Excretion Pathways
Renal Excretion: Part of the unmetabolized L-alanyl-L-cystine and its metabolites, such as sulfate, will pass through the kidneys by filtration and enter the urine, being excreted from the body with the urine. The kidneys have a certain reabsorption capacity for amino acids and peptides. However, if the concentrations of L-alanyl-L-cystine and its metabolites in the blood are too high and exceed the reabsorption threshold of the kidneys, more substances will be excreted from the urine.
Intestinal Excretion: A small amount of L-alanyl-L-cystine may enter the intestine with the secretion of bile, and the part that is not absorbed by the intestine will be excreted from the body with the feces. In addition, intestinal bacteria may also decompose and metabolize it, generating some metabolites, which may also be excreted with the feces.
