Branched-Chain Amino Acid-Free Parenteral Nutrition in the Treatment of Acute Metabolic Decompensation in Patients with Maple Syrup Urine Disease

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Abstract
MAPLE syrup urine disease is a rare inborn error of branched-chain amino acid and branched-chain keto acid metabolism due to decreased branched-chain α-keto acid dehydrogenase-complex activity.1 The branched-chain keto acids, which are the substrates in this irreversible enzyme reaction, are derived from transamination of the branched-chain amino acids leucine, isoleucine, and valine. Consumption of branched-chain amino acids in the form of dietary protein in excess of an age-dependent daily requirement for protein synthesis results in the accumulation of branched-chain amino acids and branched-chain keto acids in the tissues and body fluids of patients in whom there is little or no residual enzyme activity. The increase in levels of leucine is always more pronounced than the increase in isoleucine or valine, largely because leucine is the predominant branched-chain amino acid in most animal and plant proteins. Growth failure and psychomotor retardation are often the consequence of the chronic metabolic imbalance. Infection or fasting commonly results in acute metabolic decompensation with increased plasma levels of branched-chain amino acids and branched-chain keto acids due to the breakdown of endogenous protein, and is often associated with metabolic acidosis, ketosis, anorexia, emesis, and a potentially fatal encephalopathy.

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