Can glycine be transformed into threonine at nonketonic hyperglycinemia?

Abstract

In the forties of the XX century, it was stated by Rose what amino acids are essential that is they are not synthesized in the organism of man and animals if they cannot be found in food. Though threonine is considered an essential amino acid, nevertheless as far back as the eighties of the last century many papers and theses which described ways of mutual transformation of threonine and glycine, catalyzed by threonine aldolase, were published, which by definition contradicts the essentiality of threonine. In particular, an increase in threonine content in the tissues of human beings who were diagnosed with nonketotic hyperglycinemia was explained by its synthesis from glycine under the influence of threoninealdolase. Some authors even ascribed to serine hydroxymethyltransferase, an enzyme which catalyzes mutual transformation of two non-essential amino acids, serine and glycine, the identity to threonine aldolase. That was the reason why the view that threonine disintegrates under the action of serine hydroxymethyltransferase appeared. Later it was determined that threoninealdolase which is very active in bacteria is not present in animals’ tissues and that serine hydroxymethyltransferase does not affect threonine in mammals; thus in mammals the aldol cleavage of threonine is impossible, to say nothing about its reversibility. After that it seems there must not be any doubt that the synthesis of threonine in animals is impossible as both enzymes which catalyze disintegration of threonine (threonine dehydratase and threonine dehydrogenase) split threonine irreversibly; the fact has been known since the discovery of the enzymes. However, recently some papers in which transformation of glycine into threonine in human beings and mammals is ascribed to threonine dehydrogenase have appeared. It should be noted that threonine dehydrogenase in humans is absent. In the present paper impossibility of transformation of glycine into threonine in human beings and mammals is stated on the biochemical level which agrees with the fact that threonine is an essential amino acid.