The components of an alpha-glycerophosphate cycle and their relation to oxidative metabolism in the lens

Purification and characterization of human glycerol 3-phosphate dehydrogenases (mitochondrial and cytosolic) by NAD+/NADH redox method

Glycerol 3-phosphate (G3P) shuttle is composed of mGPDH and cGPDH and serves as the interface between carbohydrate- and lipid-metabolism. Recently, these metabolic enzymes have been implicated in type II diabetes mellitus but the detailed kinetic parameters and crystal structure of human mGPDH is unknown, though fewer studies on cGPDH are available. To characterize these enzymes, the human mGPDH and cGPDH genes were optimized and cloned into the pET-SUMO vector and pET-24a(+) vector, respectively, and over-expressed in Escherichia coli BL21 (DE3). However, SUMO-mGPDH was expressed as inclusion bodies. Hence, various culture parameters, solubilizing agents and expression vectors were used to solubilize the protein but they did not produce functional SUMO-mGPDH. Over-expression of SUMO-mGPDH along with molecular chaperone (pG-KJE8) produced a functional SUMO-mGPDH. The functional SUMO-mGPDH was purified and characterized using NAD+/NADH redox method. cGPDH was also over-expressed and purified for its characterization. DLS analysis and CD spectra of the purified proteins were performed. The mGPDH was a monomeric enzyme with MW of ∼74 kDa and displayed optimal activity in the Tris-HCl buffer (pH 7.4); while, cGPDH was a homodimer with a monomeric MW of ∼37 kDa and showed optimal activity in imidazole buffer (pH 8.0). The Kmapp was 0.475 mM for G3P, and 0.734 mM for DHAP. These methods may be used to characterize these enzymes to understand their role in metabolic disorders.

L-alpha-glycerophosphate binding to bovine gamma-crystallin: a potential link between metabolism and supramolecular order

The highly selective nature of protein-ligand interactions provides a sensitive mechanism for the modulation of cellular activity by proteins. In the eye lens the supramolecular order of the lens crystallins, which is expected to be susceptible to protein electrostatic charge, in part defines transparency. The binding of charged ligands to proteins is one way of achieving an alteration in protein electrostatic charge. Evidence is presented that L-alpha-glycerophosphate, a major phosphorus metabolite of eye lens metabolism, binds to the globular protein, gamma-crystallin with moderately high affinity and in a positive cooperative manner. The following binding parameters were obtained from equilibrium measurements: minimum number of binding sites, n = 2; Kassoc = 6.2 +/- 0.5 x 10(3) M-1; cooperativity parameter, alpha H = 1.9 +/- 0.1. Interactive computer graphics display techniques were used to locate putative ligand binding sites, and in turn, to identify the possible molecular interactions responsible for the binding of ligand to protein at one of the sites. One putative binding site was located in the cleft between the two domains of gamma II-crystallin. Arginyl residues 79 and 147 are involved in ligand binding as are the peptide carbonyl oxygens of residues Tyrosyl-50 and Aspartyl-156. Five hydrogen bonds between the ligand and the protein structure are predicted for the binding of L-alpha-glycerophosphate, whereas only 3 occur for the binding of the "unnatural" D-enantiomorph. Modulation of both lens protein supramolecular organization and lens metabolism is predicted to be a consequence of L-alpha-glycerophosphate binding to gamma-crystallin in the lens.

The effect of prolonged elevated glucose levels on the phosphate metabolism of the rabbit lens in perfused organ culture

13C and 31P nuclear magnetic resonance (NMR) techniques were used to monitor phosphate metabolite longitudinal (T1) relaxation times and metabolism, the sorbitol pathway, and related bioenergetic processes in cultured rabbit lenses through 9 days of incubation with constant perfusion. Lenses were incubated in a modified TC-199 medium containing either 5.5- or 35.5 mM [1-13C]-enriched glucose. The NMR studies were augmented by biochemical and cation analyses, and by the visual assessment of lens clarity. In the hyperglycemic rabbit lens, relative to normal values, longitudinal (T1) relaxation times for phosphorus metabolites increased from 33- to 50% [with the exception of inorganic phosphate (Pi)]. This provides the first documentation that a pathophysiological insult to the lens can alter phosphorus metabolite T1 values. The determination of steady state levels for the NMR visible phosphorus metabolites present in the lens was obtained after correction for T1 differential saturation effects, and normalization to reflect the content of phosphorus equivalents in each metabolite. Relative to control lenses (i.e. incubated in 5.5 mM glucose-containing medium) the NMR visible phosphate metabolite pool of rabbit lenses subjected to a hyperglycemic stress for an extended period of time (greater than 72 hr) is characterized by the following statistically significant differences: a 23% decrease in the mean level of ATP; a 51% increase in the mean level of alpha-glycerolphosphate (alpha GP); a 56% decrease in the mean level of Pi; the appearance of an unidentified resonance at 6.2 ppm after 115- to 120-hr incubation; and lenticular acidification of 0.10 to 0.17 pH units. No statistically significant differences in the mean levels of ADP, dinucleotides (predominantly NAD+-NADH), and phosphomonoesters (other than alpha GP) were evident. Parallel 13C NMR measurements provided a real-time confirmation of sorbitol production and accumulation in rabbit lenses incubated in 35.5 mM glucose-containing medium. In agreement with classical biochemical analysis (Chylack and Kinoshita, 1969) sorbitol production attained a plateau level after ca. 3 days of incubation. Cation determinations performed at the conclusion of the 9-day incubation indicated that the lenses incubated under the two conditions have similar Na+ and K+ levels. Rabbit lenses incubated in normal glucose medium remained clear for at least 8 days. By contrast, for the rabbit lenses incubated in elevated glucose medium, equatorial opacification became evident by day 5; by day 8 extensive bleb formation and opacification was evident.(ABSTRACT TRUNCATED AT 400 WORDS)

The metabolism of amino acids in the bovine lens. Their oxidation as a source of energy

1. The metabolism by the bovine lens of nine (14)C-labelled l-amino acids was studied. These were: alanine, aspartate, glutamate, leucine, lysine, proline, serine, tyrosine and tryptophan. 2. All were taken up by the tissue and incorporated into protein. 3. Aspartate and glutamate, although poorly taken up, were readily metabolized to CO(2). Radioactivity from glutamate was also found in glutathione, glutamine, proline and ophthalmic acid. Aspartate was converted into glutamate, glutathione, proline, alanine and lactate. 4. Alanine was largely converted into lactate, which was released into the medium, but incorporation of radioactivity into CO(2), glutamate, glutathione, aspartate and lipids also occurred. 5. Radioactivity from leucine was detected in CO(2), lipids, glutamate, glutathione, proline and glutamine. 6. Lysine was only slightly broken down by the bovine lens; radioactivity was observed in CO(2), glutamate, glutathione, proline and two unidentified compounds. 7. Proline was metabolized to glutamate from which CO(2), glutathione and glutamine were formed. Hydroxyproline in the capsule collagen was labelled. 8. Radioactivity from serine was found in CO(2), lipids, glutathione, glycine, cystine, ATP, lactate and three unidentified compounds, one of which was probably taurine. 9. Neither tyrosine nor tryptophan were metabolized by the bovine lens. 10. The ability of the lens to metabolize amino acids was also shown by measurement of NH(3) production: more NH(3) was formed when glucose was absent from the incubation medium. 11. These experiments suggest that oxidation of amino acids is a source of energy for the lens.