With stoichiometric amounts of the tetrahydropteridine, the initial rate of tyrosine formation is independent of both the sheep-liver enzyme and TPNH, a fact that makes it unlikely that either of these components is directly involved in the hydroxylation reaction. A putative cellular function of tetrahydropteridines was investigated in Dictyostelium discoideum Ax2 using a mutant disrupted in the gene encoding sepiapterin reductase.
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