Is transketolase-like protein,TKTL1, transketolase?

Until recently it was assumed that the transketolase-like protein (TKTL1) detected in the tumor tissue, is catalytically active mutant form of human transketolase (hTKT). Human TKT shares 61% sequence identity with TKTL1. And the two proteins are 77% homologous at the amino acid level. The major difference is the absence of 38 amino acid residues in the N-terminal region of TKTL1. Site-specific mutagenesis was used for modifying hTKT gene; the resulting construct had a 114-bp deletion corresponding to a deletion of 38 amino acid residues in hTKT protein. Wild type hTKT and mutant variant (DhTKT) were expressed in Escherichia coli and isolated using Ni-agarose affinity chromatography. We have demonstrated here that DhTKT is devoid of transketolase activity and lacks bound thiamine diphosphate (ThDP). In view of these results, it is unlikely that TKTL1 may be a ThDP-dependent protein capable of catalyzing the transketolase reaction, as hypothesized previously.