Quaternary structure of recombinant TK1 and the effect of reducing agent. Untreated (●) or DTE-treated (○) recombinant canine (A) and human (B) TK1 was analyzed by size exclusion chromatography. Four micrograms of freshly isolated TK1 in 200 μl buffer were injected into the column and eluted. A total of 24 fractions were collected. TK1 activity in the fractions was determined by radiochemical assay and TK1 protein was detected by immunoaffinity detection method as described in Materials and Methods. Inset: activity profile of untreated recombinant canine TK1. Arrows indicate the elution position of molecular weight markers. Western blot analysis of FPLC fractions of canine (C) and human (D) TK1; untreated (−DTE), and DTE-treated (+DTE). The numbers represent FPLC fractions.