Sulphite is not a normal component of urine. Sulphite, either ingested as a food preservative or endogenously formed, is usually oxidised to sulphate prior to its excretion by the action of sulphite oxidase, a molybdenum-dependent enzyme. Sulphite thus appears in the urine in a) sulphite oxidase deficiency (extremely rare) b) Mb deficiency (not so rare) c) asthmatic hypersensitivity after the ingestion of sulpite-preserved foods. In Mb deficiency (b) the disappearance of sulphite from the urine after Mb administration is used as a functional test for Mb deficiency.
Sulphites (which is a generic term covering Na and K sulphite, bisulphite, and metabisulphite, as well as sulphur dioxide) are widely used as food preservatives. Sulphites are, for example, sprayed on lettuce to keep it looking fresh on display in shops. Sulphur dioxide and sulphites react with the free -SH groups of cysteine in proteins and, when the protein is degraded, this eventually yields thiosulphate, which will also be excreted in the urine. The excretion of thiosulphate may also be a good indicator of a long-term exposure to sulphur dioxide and/or sulphite(s).
Sulphites can cause degradation of thiamine (vitamin B1) and the possible correlation of sulphite excretion with schizophrenia may be due to the fact that some such patients are in fact suffering from food hypersensitivity (or sulphite oxidase deficiency/Mb deficiency) to the consequent detriment of their nerve cell B1 levels
Early Morning Urine sample
Postal Samples Acceptable:
Methods to measure urinary sulphite include either (1) the Merck kit (Merckoquant Sulfitest) the basis of which is its colour reaction with sodium nitroprusside, potassium ferricyanide and zinc sulphate) or (2) the decolorisation of pararosaniline (basic fuchsin). Urine should be collected without preservative and, if there is bacterial contamination, a fresh random urine is better than a 24 hour collection. In a normal urine sulphite should be undetectable.
JIMD 6 (suppl 2) 95 (1983)
Pediatrics 73, 631 (1984)
BMJ 297, 105 (1988)
Lancet 2, 644 (1988)
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