TRANSKETOLASE IS A MAJOR PROTEIN IN THE MOUSE CORNEA

Earlier experiments in this laboratory identified a highly expressed 6 5-68-kDa protein in both mouse and human corneas (Cuthbertson, R. A., Tomarev, S. I., and Piatigorsky J. (1992) Proc. Natl. Acad. Sci. U.S.A . 89, 4004-4008). Here, we demonstrate that this protein is transketol ase (TKT; EC 2.2.1.1), an enzyme in the nonoxidative branch of the pen tose-phosphate pathway, based on peptide and cDNA isolation and sequen ce analysis of mouse cornea protein and RNA samples, respectively. Whi le expressed at low levels in a number of tissues, the 2.1-kilobase TK T mRNA was expressed at a 50-fold higher level in the adult mouse corn ea. The area of most abundant expression was localized to the cornea e pithelial cell layer by in situ hybridization. Western blot analysis c onfirmed TKT protein abundance in the cornea and indicated that TKT ma y comprise as much as 10% of the total soluble protein of the adult mo use cornea. Soluble cornea extracts exhibited a correspondingly high l evel of TKT enzymatic activity. TKT expression increased progressively through cornea maturation, as shown by Northern blot, in situ hybridi zation, Western blot, and enzymatic analyses. TKT mRNA and protein wer e expressed at low levels in the cornea prior to eye opening, while ma rkedly increased levels were observed after eye opening. Taken togethe r, these observations suggest that TKT may be a cornea enzyme-crystall in, and suggest that the crystallin paradigm and concept of gene shari ng, once thought to be restricted to the lens, apply to other transpar ent ocular tissues.