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Other Metabolic Disorders: Health Implications Of The Disorders Of Amino Acid Metabolism

Updated on February 18, 2014

Lesch-Nyhan syndrome

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A General Overview

With the development of methods to detect enzymatic defects and more advanced biochemical techniques, several metabolic disorders have been brought to light. Though many of these disorders are rare, the investigations have helped in improving the understanding of several metabolic pathways. Many of these metabolic defects are genetically inherited and therefore genetic counseling is important in their prevention. Diagnosis of these metabolic disorders early in life and avoidance of the offending articles of diet or biochemical correction of the abnormality helps in establishing normalcy. The following are a list of the important metabolic disorders.

Some metabolic disorders

Abnormalities of glycogen metabolism: Glycogenosis, glycogen storage diseases such as Type I (Von Gierke’s); Type II (Pompe’s), Type III (Cori’s, Forbe’’s), Type IV (Andersen’s); Type V (McArdle’s), Type VI (Her’s) and Type Via.

Abnormalities of purine metabolism: Gout, Lesch-Nyhan syndrome, myopathy and combined immunodeficiency.

Abnormalities of pyrimidine metabolism: Orotic aciduria

Lysosomal storage disease: Mucopolysaccharidoses (MPS) such as:

  1. MPS IH-Hurler’s syndrome (Gargoyism)
  2. MPS IS (Hunter’s syndrome)
  3. MPS III (Sanfilipo- A, B and C)
  4. MPS IV (Morquio)
  5. MPS VI (Maroteaux- Lamy)
  6. MPS VII

Mucolipidoses (ML) such as Mucolipidoses types I, II, III and IV. Mannosidosis, fucosidosis and aspartyl glucosaminuria.

Lipid storage disorders: Such as Gaucher’s disease, Nelmann-Pick disease, Farber’s lipogranulomatosis, Krabbe’s disease, Metachromatic leucodystrophy, Multiple sulphatase deficiency, Fabry’s disease, GM1 gangliosidosis, GM2 gangliosidosis, Infantile Tay-Sach’s disease and Juvenile Tay- Sach’s diseases (Sandoff’s).

Disorders Of aminoacid metabolism: Hyperphenylalaninemia (phenylketonuria- PKU), tyrosinemia, histidinemia, disorders of enzymes concerned with the urea cycle leading to increase in citrulline, argininosuccine acid and ornithine in blood. Disorders of lysine metabolism such as periodic hyperlysinemia, persistent hyperlysinemia, and hyperpipecolatemia.

Disorders of branched- chain aminoacid metabolism such as Maple-syrup-urine disease, isovaleric academia, propionic academia, methyl malonic aciduria and biotin responsive multiple carboxylase deficiency.

Disorders of sulphur- containing amino acids such as homocystinuria, cystinosis and alkaptonuria.

Disorders of fat metabolism such as hyperlipidemias, Fredrickson’s types 1, 2a, 2b, 3, 4 and 5.

Inherited disorders of mineral metabolism: Hemochromatosis and Wilson’s disease.

Disorders of heme metabolism: Porphyrias.

Phenylketonuria

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Disorders Of Amino Acid Metabolism

These are heterogenous group of rare disorders clinically characterized by mental retardation and a shortened life span, generally inherited as autosomal recessive and ranging in severity from mild to very severe forms.

Hyperphenylalaninemia (Phenylketonuria- PKU):This is one of the most widespread metabolic errors occurring in infancy. It is an autosomal recessive disorder caused by the deficiency of hepatic phenylalanine hydroxylase which is required for the conversion of phenylalanine into tyrosine. In the well developed case, there is severe mental retardation, seizures, microcephaly, hypopigmentation of skin and hair, and eczema. Ingestion of phenylalanine in milk and milk products aggravate the symptoms. Death occurs due to intercurrent infections. Phenylacetic, phenylpyruvic and phenylalanine, occur in excessive quantities. These impart a musty odour to the urine and give a green coloyr with 5% ferric chloride solution within 2 to 3 minutes.

Avoidance of milk and other phenylalanine containing diets from the first month of life prevents the development of symptoms. Even in established cases, withdrawal of dietary phenylalanine improves the symptoms.

Diagnosis: The urine is screened by ferric chloride test for metabolites of phenylalanine. The Gurthrie test which is a filter paper spot screening test for the presence of excess amounts of phenylalanine in blood. In infants, with a positive Guthrie test, quantitative estimation of aminoacids in blood is done. Serum phenylalanine is above 20 mg/dl in severe PKU, whereas normally, the levels of phenylalanine and tyrosine do not exceed 1 mg/dl.

Treatment: Dietary measures using diets which are low is phenylalanine (200 to 800 mg phenulalanine/day) help to keep the serum level of phenylalanine below 12 mg/dl. This is continued for the first 8 to 10 years of life.

© 2014 Funom Theophilus Makama

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