Isolation from erythrocytes of a green hemoprotein with ferroactivator activity

The mitochondrial isoform of phosphoenolpyruvate carboxykinase (PEPCK-M) and glucose homeostasis: has it been overlooked?

BACKGROUND

Plasma glucose levels are tightly regulated within a narrow physiologic range. Insulin-mediated glucose uptake by tissues must be balanced by the appearance of glucose from nutritional sources, glycogen stores, or gluconeogenesis. In this regard, a common pathway regulating both glucose clearance and appearance has not been described. The metabolism of glucose to produce ATP is generally considered to be the primary stimulus for insulin release from beta-cells. Similarly, gluconeogenesis from phosphoenolpyruvate (PEP) is believed to be the primarily pathway via the cytosolic isoform of phosphoenolpyruvate carboxykinase (PEPCK-C). These models cannot adequately explain the regulation of insulin secretion or gluconeogenesis.

SCOPE OF REVIEW

A metabolic sensing pathway involving mitochondrial GTP (mtGTP) and PEP synthesis by the mitochondrial isoform of PEPCK (PEPCK-M) is associated with glucose-stimulated insulin secretion from pancreatic beta-cells. Here we examine whether there is evidence for a similar mtGTP-dependent pathway involved in gluconeogenesis. In both islets and the liver, mtGTP is produced at the substrate level by the enzyme succinyl CoA synthetase (SCS-GTP) with a rate proportional to the TCA cycle. In the beta-cell PEPCK-M then hydrolyzes mtGTP in the production of PEP that, unlike mtGTP, can escape the mitochondria to generate a signal for insulin release. Similarly, PEPCK-M and mtGTP might also provide a significant source of PEP in gluconeogenic tissues for the production of glucose. This review will focus on the possibility that PEPCK-M, as a sensor for TCA cycle flux, is a key mechanism to regulate both insulin secretion and gluconeogenesis suggesting conservation of this biochemical mechanism in regulating multiple aspects of glucose homeostasis. Moreover, we propose that this mechanism may be important for regulating insulin secretion and gluconeogenesis compared to canonical nutrient sensing pathways.

MAJOR CONCLUSIONS

PEPCK-M, initially believed to be absent in islets, carries a substantial metabolic flux in beta-cells. This flux is intimately involved with the coupling of glucose-stimulated insulin secretion. PEPCK-M activity may have been similarly underestimated in glucose producing tissues and could potentially be an unappreciated but important source of gluconeogenesis.

GENERAL SIGNIFICANCE

The generation of PEP via PEPCK-M may occur via a metabolic sensing pathway important for regulating both insulin secretion and gluconeogenesis. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Interactions of methemoglobin and green hemoprotein in chemiluminescent gels

Peroxidation of luminol (5-amino-2,3-dihydrophthalazine-1,4-dione) catalyzed by human green hemoprotein (GHP) and bovine methemoglobin (MetHb) in gels composed of cross-linked bovine serum albumin was examined. The chemiluminescence (CL) was followed with a low-light intensity video camera and imaging system attached to a circularly polarized microwave guide (2450 MHz) for heating the samples from 24 degrees C to 37 degrees C. Steady-state CL was maintained in the gels for 10 min. The intensity of the CL varied with temperature. When combined with MetHb in the same gel, GHP inhibited CL of MetHb from 83.6% to 98.2% over a fiftyfold concentration range of GHP. Although MetHb/GHP combination gels were inhibited, they generated a 6.12-fold CL per degree C change compared to a 0.19-fold per degree C change for MetHb gels and a 0.31-fold per degree C change for GHP gels. The data suggest an interaction between GHP and MetHb that inhibits the CL reaction, is not interfered with by large amounts of albumin, and is partially reversed by heating.