Upregulated Phospholipase D Activity Towards Glycosylphosphatidylinositol-Anchored Proteins in Micelle-Like Serum Complexes in Diabetic/Obese Rats and Humans.

Glycosylphosphatidylinositol-anchored proteins (GPI-AP) with the complete glycolipid anchor attached are present in rat and human serum at amounts which are lower in insulin-resistant/obese rats compared to normal ones. These findings prompted further evaluation of the potential of full-length GPI-AP for the prediction and stratification of metabolically deranged states. A comparison of the signatures of horizontal surface acoustic waves which were generated by full-length GPI-AP in course of their specific capture by and subsequent dissociation from a chip-based sensor between those from rat serum and those reconstituted into lipidic structures strongly argues for expression of full-length GPI-AP in serum in micelle-like complexes in concert with phospholipids, lysophospholipids and cholesterol. Both the reconstituted and the rat serum complexes were highly sensitive towards mechanical forces, such as vibration. Furthermore, full-length GPI-AP reconstituted into micelle-like complexes represented efficient substrates for cleavage by serum glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD). Serum GPI-PLD activity towards full-length GPI-AP in micelle-like complexes, but not in detergent micelles, was positively correlated to early states of insulin resistance and obesity in genetic and diet-induced rat models as well as to the body weight in humans. Moreover, the differences in the degradation of GPI-AP in micelle-like complexes were found to rely in part on the interaction of serum GPI-PLD with an activating serum factor. These data suggest, that serum GPI-PLD activity measured with GPI-AP in micelle-like complexes is indicative of enhanced release of full-length GPI-AP from relevant tissues into the circulation as a consequence of early metabolic derangement in rats and humans.