The
glycine deportation system is an essential component of
glycine catabolism in man whereby 400 to 800mg
glycine per day are deported into urine as
hippuric acid. The molecular escort for this deportation is
benzoic acid, which derives from the diet and from gut microbiota metabolism of dietary precursors. Three components of this system, involving hepatic and renal metabolism, and renal active tubular secretion help regulate systemic and central nervous system levels of
glycine. When
glycine levels are pathologically high, as in congenital
nonketotic hyperglycinemia, the
glycine deportation system can be upregulated with pharmacological doses of
benzoic acid to assist in normalization of
glycine homeostasis. In congenital
urea cycle enzymopathies, similar activation of the
glycine deportation system with
benzoic acid is useful for the excretion of excess
nitrogen in the form of
glycine. Drugs which can substitute for
benzoic acid as substrates for the
glycine deportation system have adverse reactions that may involve perturbations of
glycine homeostasis. The
cancer chemotherapeutic agent
ifosfamide has an unacceptably high incidence of
encephalopathy. This would appear to arise as a result of the production of toxic
aldehyde metabolites which deplete
ATP production and sequester
NADH in the mitochondrial matrix, thereby inhibiting the
glycine deportation system and causing de novo
glycine synthesis by the
glycine cleavage system. We hypothesize that this would result in hyperglycinemia and
encephalopathy. This understanding may lead to novel prophylactic strategies for
ifosfamide encephalopathy. Thus, the
glycine deportation system plays multiple key roles in physiological and neurotoxicological processes involving
glycine.