Palmitate action to inhibit glycogen synthase and stimulate protein phosphatase 2A increases with risk factors for type 2 diabetes.

Recent studies have suggested that abnormal regulation of protein phosphatase 2A (PP2A) is associated with Type 2 diabetes in rodent and human tissues. Results with cultured mouse myotubes support a mechanism for palmitate activation of PP2A, leading to activation of glycogen synthase kinase 3. Phosphorylation and inactivation of glycogen synthase by glycogen synthase kinase 3 could be the mechanism for long-chain fatty acid inhibition of insulin-mediated carbohydrate storage in insulin-resistant subjects. Here, we test the effects of palmitic acid on cultured muscle glycogen synthase and PP2A activities. Palmitate inhibition of glycogen synthase fractional activity is increased in subjects with high body mass index compared with subjects with lower body mass index (r = -0.43, P = 0.03). Palmitate action on PP2A varies from inhibition in subjects with decreased 2-h plasma glucose concentration to activation in subjects with increased 2-h plasma glucose concentration (r = 0.45, P < 0.03) during oral glucose tolerance tests. The results do not show an association between palmitate effects on PP2A and glycogen synthase fractional activity. We conclude that subjects at risk for Type 2 diabetes have intrinsic differences in palmitate regulation of at least two enzymes (PP2A and glycogen synthase), contributing to abnormal insulin regulation of glucose metabolism.

Interaction of biotin with Mg-O bonds: bifunctional binding and recognition of biotin and related ligands by the Mg(15-crown-5)2+ unit.

The interaction between biotin and the macrocyclic magnesium complex Mg(15-crown-5)(Otf)2 (15-crown-5 is 1,4,7,10,13-pentaoxacyclopentadecane, Otf(-) is trifluoromethanesulfonate anion) in solution was studied as a model for metal-biotin interactions that may be important in its speciation and function. Shifts in the solution IR spectrum establish that the interaction is dominated by ligation between the carbonyl oxygen of the ureido ring of biotin and the Mg2+ cation. However, comparative binding studies using NMR spectroscopy and conductivity reveal a substantial enthalpic contribution to binding that arises from interactions between the ureido -NH moiety and the macrocyclic ring. This is interpreted in terms of a weak-to-moderate hydrogen bond formed between the -NH group and an oxygen from the crown, which is simultaneously coordinated to Mg2+. This hypothesis is reinforced by quantitative examination of the binding of N-methylated derivatives of 2-imidazolidone, which shows that N,N'-dimethylation decreases the affinity of Mg(15-crown-5)(Otf)2 for the ligand by 2 orders of magnitude. This can be understood in terms of the structure of Mg(15-crown-5)(Otf)2. It shows a pentagonal bipyramidal coordination geometry where the five equatorial positions are occupied by the macrocyclic oxygen donors. The axial positions are occupied by weakly coordinating Otf(-) anions, which are readily displaced by biotin and related derivatives. The M-O(crown) bond distance ranges from 2.1 to 2.3 A, providing structural complementarity for the 2.2 A C=O...HN- bite distance in the ureido group, which leads to strong interaction. The contribution from hydrogen bonding illustrates the importance of second-shell interactions in the biocoordination chemistry of Mg2+. These can serve to organize cofactor interactions with biomolecules, as was recently demonstrated for a biotin-selective RNA aptamer that depends on a direct biotin-magnesium interaction for recognition of biotin (Nix, J.; Sussman, D.; Wilson, C. J. Mol. Biol. 2000, 296, 1235-1244). These results are significant in the context of the observed magnesium requirement in biotin-dependent carboxylase enzymes, where noncovalent interactions with biotin may be important in its activation toward carboxylation in the first step of biotin-dependent CO2 transfer. The synthetic system presented here also suggests that the Mg-O bond may be considered a constituent design element in the rational preparation of complexes to bind and recognize biotin.