Variability in the phenotypic expression of abnormal sarcosine metabolism in a family.

A retarded child with hypersarcosinemia and his family were studied by loading tests to determine the probable site of his defect. On the basis of his response to folate treatment, a partially-reversible defect in the formation of activated formaldehyde in the reaction catalyzed by sarcosine dehydrogenase was considered to be the most likely site. During a glycine loading test, sarcosine levels in the plasma and urine increased, indicating that the direct transmethylation of glycine to sarcosine could occur in this patient. The father of the proband tolerated a load of sarcosine poorly, resembling the proband in his plasma sarcosine levels. No evidence that glycine could be transmethylated to sarcosine was found in the father, despite the fact that his peak glycine level was four times higher than the proband's. These findings provide indirect evidence that sarcosine formation may be affected by two additional components besides the apo moiety of sarcosine dehydrogenase, the availability of tetrahydrofolic acid as a one carbon unit carrier and the integrity of the transmethylase which catalyzes the direct transmethylation of glycine to sarcosine.