Hydroxypyruvate from serine

Hydroxypyruvate is primarily derived from serine through a transamination catalyzed by an alanine hydroxypyruvate transaminase. It is believed that excessive amounts of hydroxypyruvate are converted to l-glycerate (the reaction catalyzed by lactic dehydrogenase) because of a lack of D-glyceric dehydrogenase which catalyzes the conversion of hydroxypyruvate to D-glyceric acid. 

Hydroxypyruvate can be formed from serine by transamination dihy-droxyacetone is readily phosphorylated to triose phosphate. As CO is removed, more hydroxypyruvate could be formed from serine. Some such cycle would provide a nonfolic-mediated disposition of formaldehyde. Formaldehyde, in turn, could be generated by the oxidation of the W-methyl of sarcosine sarcosine in turn is generated by the removal as active formaldehyde of one of the methyls of dimethylglycine (Mackenzie and Frisell, 1968). Dimethylglycine is formed by the well-established sequence sarcosine — glycine + HCHO serine ethanolamine + COj — mono-methylethanolamine —> dimethylethanolamine —> choline —> betaine —> dimethylglycine. 

As shown on p. 168j, serine can be converted to pyruvate (elimination of water forms the unsaturated amino acid which goes via the imino acid to the a-keto acid) as well as hydroxypyruvate or phosphoserine. The phosphorylated form, which could also come from 3-phosphoglycerate, occurs in a few proteins and phospha-tides (Chapt. XIII). Other bases present in the phosphatides, i.e. ethanolamine and choline, also originate from serine. 

Amino acids can also turn into carbohydrates by several other pathways The decarboxylation of hydroxypyruvate (arising from serine by transfl.minfl.tion) yields active glycolaldehyde, which is then available for the transketolase reaction. Thus two a-carbons of serine can enter carbohydrate. 

Sallach observed the presence of a serine-hydroxypyruvate transaminase in liver and kidney of a number of mammalian species and partiaUy purified the enzyme from dog liver acetone powder. Alanine and pyruvate were specific for the transamination reaction, and transamination was shown to be reversible and to proceed in either direction by incubating serine with pyruvate or hydroxypyruvate with alanine. 

In addition, incomplete oxidation of polyols leads to production of higher valued chemicals, such as dihydroacetone, which is a valuable tanning agent, hydroxypyruvic acid, which is a flavor component and a possible starting material for DL-serine synthesis, and tartronic and mesoxalic acids, which are important intermediates for novel polymer and pharmaceutical synthesis. Therefore, research on cogeneration of electricity and higher valued chemicals from polyols... 

From glutamate the reverse path is also possible With a-keto acids from other sources the corresponding amino acids and a-ketoglutarate can be formed (the above diagram in reverse). The organism is able in this way to synthesize several amino acids alanine out of pyruvate, serine out of hydroxypyruvate, and, of course, the two dicarboxylic acids already mentioned, glutamic and aspartic acid. 

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