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Neurological dysfunction is an important manifestation of inherited metabolic disorders.
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- 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
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