Homocystinuria is due to a lack of the enzyme cystathione- beta-synthase which usually convert methionine to cystathione. This results in accumulation of the metabolic intermediates homocysteine and homocystine (referred to as homocyst(e)ine). Homocystine is excreted in the urine.
It is the homocyst(e)ine which causes the complications of homocystinuria.
Reference
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