Fig. 2.

(A and B) Site-specific ¹²C/¹³C isotope fractionation (‰) of the glucose isomerase (A) and the invertase (B) reactions assayed in vitro. Asterisks indicate fractionations significantly different from zero (P < 0.05). For invertase, there is no thermodynamic measurement because the reaction is irreversible. In both cases, fractionation values are relative to that in C-4; that is, it was assumed that no fractionation occurred at the C-4 position because no net chemical event takes place at this site. Positive values indicate a fractionation against ¹³C, and negative values indicate a fractionation in favor of ¹³C.