Amino acids
- An insight into the biochemistry of inborn errors of metabolism for a clinical neurologist

Analysis of amino acids in various physiological fluids such as plasma, urine and cerebrospinal fluid is central to the investigation of a possible neurometabolic disorder. Some laboratories include a selection of relatively nonspecific chemical tests for the presence of compounds containing certain functional groups such as disulfides by using the cyanide nitroprusside test, etc. However, the presence of large amounts of interfering compounds sometimes makes these methods unreliable. Paper or thin layer chromatography of amino acids in plasma and urine and visualisation by treatment with ninhydrin is useful for detecting excess amino acids such as phenylalanine in phenylketonuria. Quantitative amino acid analysis by ion-exchange chromatography, high-pressure liquid chromatography (HPLC) or tandem mass spectrometry (MS-MS) provides confirmation of the identity and concentrations of the amino acids and provides accurate information on the levels of amino acids that may be present in subnormal concentrations. These methods are also necessary to quantitate reliably the concentration of amino acids in fluids such as CSF.

In inborn errors of specific amino acid metabolism, marked increases are observed in plasma and urine, which are sufficiently specific to suggest a diagnosis. Secondary abnormalities of amino acid concentrations in plasma and urine are also very common. Severe hepatocellular disease, renal tubular disease, catabolic states, malnutrition, malignancy, infections, pregnancy, vitamin deficiencies, burns and other injuries are all associated with the disturbances of amino acid concentrations in plasma, urine or both. Increased amino acid levels in the urine in the absence of corresponding increases in plasma levels occur generally due to inherited or acquired renal transport defects. An important point to be considered is that the concentrations of various amino acids in plasma depend on the metabolic state of the individual. During the postprandial period, the levels of essential amino acids, phenylalanine, tyrosine, lysine, valine, leucine, isoleucine, etc, are increased. Prolonged fasting will result in an elevation of the branched-chain amino acids, leucine, isoleucine and valine. The reference values are based on plasma collected 4-6 h after the last meal.

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