Evidence for a posttranslational covalent modification of liver glyceraldehyde-3-phosphate dehydrogenase in hibernating jerboa (Jaculus orientalis)

The specific activity of d-glyceraldehyde-3-phosphate (G3P) dehydrogenase (phosphorylating) (GPDH, EC 1.2.1.12) found in liver of induced hibernating jerboa (Jaculus orientalis) was 2–3-fold lower than in the euthermic animal. However, the comparative analysis of the soluble protein fraction of these tissues by SDS-PAGE and Western blotting showed no significant changes in the intensity of the 36 kDa protein band of the GPDH subunit. After using the same purification procedure, the GPDH from liver of hibernating jerboa exhibited lower values for both apparent optimal temperature and specific activity than the enzyme from the euthermic animal. Similar non-linear Arrhenius plots were obtained, but the E_a values calculated for the GPDH from hibernating tissue were higher. Although in both purified enzyme preparations four isoelectric GPDH isoforms were resolved by chromatofocusing, those of hibernating liver exhibited more acidic pI values (pI 7.3–6.1) than the hepatic isoforms of euthermic animals (pI 8.7–8.1). However, all liver GPDH isoforms exhibited similar native and subunit molecular masses and cross-reacted with an antibody raised against muscle GPDH. The comparison of the kinetic parameters of both purified preparations and the main isoforms isolated from euthermic and hibernating tissues showed the decreased catalytic efficiency of hibernating enzyme being exclusively due to a lower V_max for both substrates G3P and NAD⁺. Phosphodiesterase treatment of cell-free extracts increased GPDH activity in the case of hibernating liver only. The pI of the main isoform purified from this tissue, about 6.9, changed after this treatment to an alkaline value (pI 8.44) similar to those of the euthermic GPDH isoforms. Differential ultraviolet absorption spectra of these isoforms indicated that a substance absorbing at 260 nm, that was released by the phosphodiesterase digestion, was present in the enzyme of hibernating tissue. Incubation of purified GPDH with the NO-releasing agent sodium nitroprussite produced under conditions that promote mono-ADP-ribosylation a dramatic decrease of activity (up to 60%) of both euthermic and phosphodiesterase-treated hibernating preparations but only a marginal inhibition of the hibernating enzyme. These data suggest that liver GPDH of hibernating jerboa exhibits a posttranslational covalent modification, being probably a mono-ADP-ribosylation. The resulting inhibition of enzyme activity could contribute to the wide depression of the glycolytic metabolic flow associated with mammalian hibernation.     

Comment on Gallistel: Behavior theory and information theory: Some parallels

In this article, Gallistel proposes information theory as an approach to some enduring problems in the study of operant and classical conditioning.