Amino acid depletion in the blood serum is currently being exploited and explored for therapies in tumors or viral infections that are auxotrophic for a certain amino acid or have a metabolic defect and cannot produce it. The success of these treatments is because normal cells remain unaltered since they are less demanding and/or can synthesize these compounds in sufficient amounts for their needs by other mechanisms. Areas covered: This review is focused on amino acid depriving enzymes and their formulations that have been successfully used in the treatment of several types of cancer and viral infections. Particular attention will be given to the enzymes L-asparaginase, L-arginase, L-arginine deiminase, and L-methionine-γ-lyase. Expert opinion: The immunogenicity and other toxic effects are perhaps the major limitations of these therapies, but they have been successfully decreased either through the expression of these enzymes from other organisms, recombination processes, pegylation of the selected enzymes or by specific mutations in the proteins. In 2006, FDA has already approved the use of L-asparaginase in the treatment of acute lymphoblastic leukemia. Other enzymes and in particular L-arginase, L-arginine deiminase, and L-methioninase have been showing promising results in vitro and in vivo studies.

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An upshift of the growth temperature from 26 to 40 degrees C in the presence of calcium leads to the aggregation of Kluyveromyces marxianus cells and to the formation of flocs. Analysis of cell wall proteins, either by sodium dodecyl sulphate-polyacrylamide gel electrophoresis of extractable mannoproteins or by immunolocalization, revealed an accumulation of a protein with Mr 37 kDa(p37), upon flocculation. Immunological studies confirmed the homology of this protein with the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). When mRNA isolated from cells growing at 40 degrees C was translated in vitro, a 35 kDa newly labelled protein was synthesized and immunoprecipitation assays showed that this protein is recognized by p37-antiserum, suggesting that the 35 kDa polypeptide might be an unglycosylated precursor for of p37. The results indicated that the presence of this cell wall mannoprotein closely related to GAPDH is dependent on the growth temperature, suggesting its role as adhesin.

Three glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes from Kluyveromyces marxianus were identified and characterized. The coding region of two of them (GAP2 and GAP3) is very similar (99.6% homology). The other gene (GAP1) is only 86% homologous to GAP2 or GAP3 and is responsible for the expression of Gap1p. This protein is extremely homologous to the K. marxianus cell wall protein p37, presumably involved in flocculation. However, no leader sequence could be detected in this gene. The identification of the three genes was possible with the use of polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP), as it permits us to overcome the difficulties caused by the high homology amongst the genes. Expression of the GAPDH genes under different carbon sources (glucose or ethanol) was assessed either by Northern blot or reverse transcription-PCR-SSCP analysis, revealing that genes GAP1 and GAP2, but not GAP3, are transcribed. The results also indicate that the transcription of the gene encoding the cell wall protein p37 (Gap1p) is not dependent on the carbon source, in contrast with the expression of the gene GAP2, which is affected in cells growing in a glucose-depleted medium.