A search of the human genomic sequences for the TDH gene identified only one candidate location on chromosome 8p23-22. This is the expected site of the human TDH gene since there are a number of genes that neighbour the TDH genes in the genomes of human, mouse and puffer fish that are found in common in this locus. They are the myotubularin related protein 8, the B-lymphoid tyrosine kinase and the hypothetical protein C8orf13. There are differences in the gene order and orientation in the human and puffer fish TDH loci indicating that there have been re-arrangements in this locus since the divergence of the human and puffer fish lineages .
The human TDH gene contains 3 mutations that disrupt the gene transcription and translation. There is a loss of the acceptor splice sites preceding exons 4 and 6 and an in-frame stop codon in exon 6. A database search of expressed human sequences identified other TDH cDNAs and ESTs similar to the 2 cDNA sequences described. The sequence of the human cerebellum full-length cDNA clone FLJ25033 (AK057762, NEDO human cDNA sequencing project University of Tokyo, Japan) has 2 5'UTR exons, the first of which corresponds to exon -1 on the mouse genomic DNA sequence . This clone skips both exons 4 and 6, and includes the polyadenylation site. The ORF would encode a truncated 180 residue protein. Three very similar human ESTs (AI005002, AI243637 and AI809781) have exon 4 skipped and use the cryptic acceptor splice site in exon 6 that was found in clone 2 and possesses the in-frame stop codon.
The human TDH gene is a pseudogene because in all individuals examined the gene contains 1 or more mutations that disrupt RNA splicing which give rise to a variety of mRNAs in different individuals. In both pig and mouse TDH cDNAs there was no evidence of alternatively splicing . In humans, arginine-214 is replaced by a stop codon. On translation all the human mRNAs would generate a variety of truncated proteins ranging from 157 to 230 residues, some of which would also contain deletions of residues 96–146; whereas the size of the core of TDH enzyme (residues 48–364) differs by only 2 residues in other species, including some bacterial TDH proteins . The human truncated proteins would be non-functional since they would be unable to make appropriate contacts with the substrates, L-threonine and NAD+. They have lost most of the carboxy-terminal domain, which by homology with GALE [26–29], would be expected to bind L-threonine and also have lost various regions of the NAD+ binding motif that extends from exon 1 to exon 6. In comparison, even single point missense mutations in the human GALE gene disrupt protein function resulting in epimerase-deficiency galactosemia .
At present, no known human genetic disease has been associated with defects in the threonine dehydrogenase and 2-amino-3-ketobutyrate coenzyme A ligase biochemical pathway. This may be because the enzymatic activity of L-serine/threonine dehydratase is sufficient to metabolise L-threonine in humans. The mitochondrial threonine dehydrogenase enzyme is thought to act in the maintenance of free somatic threonine concentration derived from dietary threonine  suggesting that humans may not be able to regulate the level of circulating threonine as well as other mammals. In vertebrates, L-threonine is degraded by two major enzymatic pathways and attempts to determine their relative contribution in humans has been by indirect methods. Zhao et al. measured threonine oxidation by the production of labelled CO2 in adult humans from labelled threonine and concluded that the conversion of threonine to glycine through the threonine dehydrogenase pathway was not important if it exists at all . More recently, Darling et al. used a more sensitive method to measure the production of labelled glycine and CO2 and suggested that there was some threonine catabolism via the threonine dehydrogenase pathway, but it accounted for only 7–11% of total . Direct demonstration of the absence of TDH enzymatic activity in human tissues such as liver remains to be demonstrated. In contrast, in normally-fed pigs and rats 80 to 87% of L-threonine catabolism occurs via threonine dehydrogenase [15, 16]. Since the TDH gene is not functioning in humans, what enzyme(s) could be responsible for the small percentage of threonine catabolism to glycine? In bacteria, threonine aldolase also yields glycine, but mammals are thought to lack the "genuine" threonine aldolase . However, serine hydroxymethyltransferase exhibits a low threonine aldolase activity in mammalian liver .
Whether the first dehabilitating mutation was the loss of the splice acceptor site preceding exon 6 or the arginine-214 to stop codon mutation is unknown, nor is it known when in human evolution that a functional TDH gene was lost. However, the arginine codon CGA is used in only 11.1% of arginine residues in humans, being prone to mutate to a stop codon. The TDH gene has been conserved throughout evolution, being found in bacteria and other mammals , so why has it not been conserved in man? How could such a mutation establish its self in the ancestral population? It is likely that the mutation must have conferred some selectable advantage, under the prevailing environmental conditions, on those individuals who carried it, and arose at a time when the ancestral population was small. Under conditions of protein starvation, threonine dietary intake could have been a limiting factor on growth, survival and successful reproduction and a reduction in the rate of threonine catabolism would have conferred a selective advantage on those individuals with defective TDH gene. Additionally, if we consider that the threonine catabolism pathway is also a glycine synthesis pathway, then a reduction in the production of glycine in inhibitory glycinergic neurons  may have contributed to human neural evolution.
Since humans lack a functioning threonine dehydrogenase enzyme and human parasites such as trypanosomes do possess one (Edgar and Horn, unpublished results, GenBank AF529241), the parasite threonine dehydrogenase enzyme is a potential target for therapeutic intervention. Indeed, in trypanosomes such as Trypanosoma brucei, which causes sleeping sickness, L-threonine dehydrogenase is an important metabolic enzyme and inhibition of this enzyme by a wide range of sulphydryl reagents, such as tetraethylthiuram disulphide, leads to a loss of trypanosome viability [38–40].
Loss of other metabolically important genes in man results in disease susceptibility. Man and primates are scurvy-prone, having lost the L-gulono-gamma-lactone oxidase gene that is found in most other eukaryotes, and are unable to synthesize L-ascorbic acid . Man and primates have also lost the urate oxidase enzyme that catalyses the conversion of uric acid to allantoin. This results in a high concentration of uric acid in the blood, predisposing man to hyperuricemia that can lead to gouty arthritis and renal stones . Humans have recently lost 2 functional genes involved in sialic acid function. They are the CMP-N-acetylneuraminic acid hydroxylase gene (CMAH)  and the siglec-like molecule (Siglec-L1) . However, no diseases have yet been associated with the loss of these genes. The CMAH enzyme converts the sialic acid, N-acetylneuraminic acid to N-glycolylneuraminic acid, potentially affecting recognition by a variety of endogenous and exogenous sialic acid-binding lectins. Siglecs are immunoglobulin superfamily member lectins that selectively recognize different sialic acids types and siglec-L1 preferentially recognizes N-glycolylneuraminic acid. To date, no disease has been associated with the loss of the TDH gene, but humans may not be able to metabolise high protein diets as efficiently as other mammals.