Amino acids have many functions in the body. Theyare the building blocks for all body proteins-structural proteins that build muscle, connectivetissues, bones and other structures, and functionalproteins in the form of thousands of metabolicallyactive enzymes. Amino acids provide the body withthe nitrogen that is essential for growth andmaintenance of all tissues and structures.
Aside from these general functions, individual aminoacids also have specific functions in many aspects ofhuman physiology and biochemistry. L-arginine is aconditionally essential dibasic amino acid. The body isusually capable of producing sufficient amounts ofarginine, but in times of physical stress, endogenoussynthesis is often inadequate to meet the increaseddemands.
L-arginine can either be used for glucose synthesis orcatabolized to produce energy via the tricarboxylic acidcycle. It is also the sole source of nitric oxide (NO), viathe enzyme nitric oxide synthase. NO can affect avariety of physiological processes, including relaxationof arterial smooth muscle, platelet aggregation, andneuroendocrine secretion.
L-arginine is required for the synthesis of creatinephosphate. Similar to adenosine triphosphate (ATP),creatine phosphate functions as a carrier of readilyavailable energy for contractile work in muscles.Adequate reservoirs of creatine phosphate are necessaryin muscle as an energy reserve for anaerobic activity. Larginineis also a precursor of polyamines, includingputrescine, spermine and spermidine. Spermine andspermidine interact with DNA, act as physiologicalgrowth regulators of cell proliferation, and are involvedin the stabilization of cell membranes and cellorganelles. L-arginine is a potent stimulator of insulin,glucagon, and growth hormone release, and functions asa representative signal to the endocrine system thatdietary protein ingestion has taken place.
Supplemental dietary ornithine can serve as a precursorof arginine. Both ornithine and arginine have anaboliceffects in surgical trauma patients, and promote insulinas well as growth hormone secretion. Some scientistssuggest that supplementation with these amino acidsmay improve functioning of the gastrointestinal tract,perhaps via increased secretion of bioactive polyamines.Lysine is required for collagen cross-linking. Collagencross-linking is important for resiliency and elasticityof the collagen and elastin present in all connectivetissues and blood vessel walls. During formation ofnew collagen, fibroblasts secrete immature collagenstrands and a vitamin C- and copper-requiring enzyme,lysyl oxidase. Lysyl oxidase oxidizes the free aminogroup of the immature collagen's lysyl side chains.Once oxidized, these lysyl side chains spontaneouslyengage in various reactions between collagen strandsto bring about cross-linking. As a result, a complexnetwork of collagen strands is formed, providingelasticity and resiliency.
Another important role of lysine is its precursorfunction for L-carnitine. L-carnitine is necessary forfatty acid metabolism and energy production in cardiacand skeletal muscle. It is involved in fatty acidoxidation as part of the carnitine shuttle. L-carnitineshuttles fatty acids from the cytosol (the cell fluid) intothe mitochondria (the cell's powerhouses) foroxidation and energy production. Dietary lysine ispresent in the form of proteins, mainly from dairy andanimal origin. Vegetarian diets tend to provide littlelysine, because vegetable proteins, including legumes,are often low in lysine.