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.
