Assays / Genetic Enzymes / Cystathionine b-Synthetase

Principle Of Method: The reaction measured is the condensation of homocysteine with serine to form cystathionine.

[¹⁴C]-labelled serine is used in the reaction in the presence of unlabelled homocysteine and the co-factor pyridoxal phosphate. After the incubation the [¹⁴C]-cystathionine formed is separated by paper chromatography and determined by liquid scintillation counting.

Uses And Limitations Of The Method: The assay is used to diagnose synthase deficiency homocystinuria (referred to as classical homocystinuria). This is the commonest form of homocystinuria and is characterised biochemically by the presence of homocystine in urine and elevated methionine and homocystine in plasma. Rarer forms of homocystinuria, for which the laboratory does not offer confirmatory enzymology, arise from deficiency of remethylation of homocysteine to methionine which may be due to a number of causes, genetic and non-genetic. In these there is usually a low plasma methionine level.

Classical homocystinuria may be an under-diagnosed disorder because of the problems some routine laboratories have in identifying homocystine in urine. Assay of the enzyme in fibroblasts (white cells cannot be used) is thus an important procedure. Prenatal diagnosis of homocystinuria is possible by assay of the enzyme in cultured amniotic cells or cultured chorionic villi, but not by assay of villi directly.

Specimen Requirements: Fibroblasts cultured from a skin biopsy are needed. Biopsy material should be collected aseptically into a sterile bottle containing tissue culture medium (available from the laboratory), and sent at room temperature to arrive at the laboratory within 24 hours.

Biopsies for tissue culture should not be frozen.

Fibroblast cultures established in other laboratories should be sent in plastic 25 cm² flasks filled with medium.

The Laboratory Recommends Use Of A Courier Service Or Royal Mail Special Delivery For Sending All Specimens To The Laboratory.

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