Availability of cysteine and of L‐2‐oxo‐thiazolidine‐4‐carboxylic acid as a source of cysteine in intravenous nutrition

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Abstract
Cysteine is usually not included in amino acid solutions for intravenous nutrition for two reasons: it is not considered to be an essential amino acid, and it causes stability problems in solutions. The cysteine content of a solution decreases markedly in the presence of oxygen and/or glucose due to formation of various derivatives, such as cystine and D-glucocysteine. It is not clear if the formed derivatives are available for metabolic processes. A cysteine analogue, L-2-oxothiazolidine-4-carboxylic acid (OTCA), is stable in solutions and can be metabolized in the body-yielding cysteine. The metabolic availability of cysteine and OTCA from an all-in-one total parenteral nutrition (TPN) mixture was evaluated in the rat and compared to a regimen with cysteine infused from a separate container. A cysteine-free TPN mixture served as a control. All mixtures contained a suboptimal amount of methionine, which can normally be metabolized to cysteine, to enhance the nutritional effect of cysteine or OTCA supplement. The cysteine content in the admixture decreased approximately 40% during 30 hr after mixing. Nevertheless the utilization of this admixture was identical to the regimen supplying cysteine separately. The admixture containing OTCA showed similar results as the two cysteine regimens. All three regimens promoted growth significantly better than the cysteine-free-mixture. The results suggest that 1) cysteine is available for metabolic processes from its derivatives formed in an all-in-one TPN mixture, and 2) OTCA acts as a precursor of cysteine to the rat when supplied as a part of TPN.