Structural and Genetic Relationships of Human Triosephosphate Isomerase Isozymes.

Triosephosphate isomerase (TPI; EC 5.3.1.1) and deamidated forms are constitutively expressed in all cells and tissues examined in humans as well as in other primates. A second isozyme, TPI-2, is observed in a wide range of normal and transformed human cells. Moreover, the expression of TPI-2 is restricted to hominoids (apes and man), and not observed in non-hominoid primates. The TPI-1 and TPI-2 isozymes expressed in cultured human lymphoblasts from an individual who is heterozygous for a rare erythrocyte TPI variant were identically altered in their electrophoretic mobilities. Thus, the appearance of TPI-2 is due to a specific modification of the primary TPI protein, rather than the expression of a second structural gene for TPI. A procedure was developed for the isolation of the individual TPI isozymes from cultures of human lymphoblastoid cell lines. The identity and purity of the isolated proteins were confirmed by partial Edman degradation of the radiolabeled (('35)S-methionine) isozymes. Pulse-chase experiments yield similar rates of degradation for TPI-1 (0.020 hr('-1)) and TPI-2 (0.026 hr('-1)). Continuous incorporation of the radiolabel confirmed that the differences in in vivo activity levels (4.0:1.0) are regulated by the relative rates of synthesis (4.7:1.0). The absence of a precursor-product relationship suggest that the modification occurs before or during translation of the mRNA. Neither isozyme is modified by in vivo phosphorylation, N-terminal acetylation or extensive proteolysis. Spontaneous deamidation of labile asparagines is also ruled out on the basis of turnover kinetics and restricted phylogenetic appearance. Modification at several levels of cellular regulation still remain as possibilities to account for TPI-2 expression. Differential transcription of the TPI structural gene or RNA splicing may occur. Alternatively, a change in TPI structure during primate evolution may provide a site for nascent protein modification. In either possibility, the restricted expression of TPI-2 implies the modification is (1) a regulated process of development, and (2) under genetic control.PhDGeneticsUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159168/1/8304476.pd