Why methionine decreases in non classical homocystinuria?

why methionine decreases in non classical homocystinuria?

In non-classical homocystinuria, there is impaired re-methylation of homocysteine to methionine. This can be caused by defective methionine synthase or reduced availability of two essential cofactors, 5-methyltetrahydrofolate, and methylcobalamin.

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