The interconversion of serine and glycine: role of pteroylglutamic acid and other cofactors

Abstract

A soluble enzyme preparation from pigeon liver catalyzed the formation of serine-2-C14 from L-serine and glycine-2-C14. D-Serine was inactive and no C14 was incorporated into C-3 of serine. The mechanism of this reaction was elucidated by the demonstration of net synthesis of serine-C14 from formaldehyde and glycine-C14, and of net conversion of serine 2-C14 into glycine C14 and formaldehyde in presence of the enzyme. In presence of the enzyme, HC14HO and glycine were converted into serine-3-C14, but no synthesis of serine occurred when formate was substituted for formaldehyde. For synthesis of serine-2-C14 from glycine-2-C14 and L-serine, well-dialyzed enzyme preparations had to be activated by the addition of pteroylglutamic acid (PGA) and a boiled extract of yeast or liver. The yeast extract could be replaced by a combination of adenosine triphosphate (ATP) and diphosphopyridine-nucleotide (DPN). Ascorbic acid increased the formation of serine-2-C14 from glycine-2-C14 and L-serine by the liver enzyme system, and the reaction was much faster under anaerobic conditions than in the presence of O2. For net serine synthesis from glycine and formaldehyde and for net serine degradation to glycine and formaldehyde, dialyzed enzyme preparations were fully activated by the addition of PGA alone. The effect of leucovorin (N5-formyltetrahydro-PGA) in activating the crude dialyzed enzyme for serine 2-C14 formation from glycine 2-C14 and L-serine was similar to that of PGA; simultaneous addition of yeast extract was necessary for full activation. Heating dialyzed enzyme preparations to 53[degree] did not diminish their activity when supplemented by PGA and yeast extract, but activity in the presence of leucovorin was decreased to 10-20% of its former value. Of the PGA derivatives tested, tetrahydro-PGA was most effective in activating dialyzed enzyme for serine-glycine interconversion, and it activated without addition of yeast extract or other factors. Moreover, serine-glycine interconversion catalyzed by the enzyme and tetrahydro-PGA commenced at a high rate, but when PGA and yeast extract activated the enzyme there was a lag of over an hour before appreciable serine-glycine interconversion occurred. In this period enzymic reduction of PGA to tetrahydro-PGA is believed to have occurred. 4-Amino PGA inhibited serine-glycine interconversion when the enzyme preparation was supplemented with PGA or N10-formyl PGA, but only slightly when leucovorin or tetrahydro-PGA were added as supplements. The significance of these results is discussed and a scheme describing the role of PGA derivatives in one-carbon metabolism is proposed, in which tetrahydro-PGA is presumed to play a central part in the transfer of one-carbon units.