The Case for De Novo Production of Thiaminase By Alewife

Thiamine (vitamin B₁) deficiency causes embryo mortality in fish. In the Great Lakes, thiamine deficiency is an impediment to restoration of native lake trout (Salvelinus namaycush) stocks. The non-native fish alewife (Alosa pseudoharengus), a preferred prey item for lake trout, contains elevated levels of type I thiaminase, an enzyme that degrades thiamine. The source of thiaminase in alewife has been controversial. We present evidence consistent with the hypothesis that alewife have a gene encoding thiaminase and produce thiaminase protein de novo. We first identified a gene in zebrafish (Danio rerio) with homology to a partial protein sequence of thiaminase isolated from red cornetfish (Fistularia petimba). When the zebrafish putative thiaminase homolog was expressed in E. coli cells, the isolated recombinant protein had thiaminase activity. A homolog to the zebrafish thiaminase gene was identified in alewife. The size and isoelectric point predicted for the protein encoded by the putative alewife thiaminase gene match the size and isoelectric point of thiaminase isolated from alewife tissue. Surprisingly, the putative fish thiaminase protein sequences show a weak but significant alignment to bacterial TenA, a type II thiaminase that acts in a thiamine salvage pathway. The physiological function of thiaminase produced by fish is unknown.