Grant B. McClelland

Peroxisomal membrane monocarboxylate transporters: evidence for a redox shuttle system?

One of the many functions of liver peroxisomes is the beta-oxidation of long-chain fatty acids. It is essential for the continuation of peroxisomal beta-oxidation that a redox shuttle system exist across the peroxisomal membrane to reoxidize NADH. We propose that this redox shuttle system consists of a substrate cycle between lactate and pyruvate. Here we present evidence that purified peroxisomal membranes contain both monocarboxylate transporter 1 (MCT 1) and MCT 2 and that along with peroxisomal lactate dehydrogenase (pLDH) form a Peroxisomal Lactate Shuttle. Peroxisomal beta-oxidation was greatly stimulated by the addition of pyruvate and this increase was partially inhibited by the addition of the MCT blocker alpha-cyano-4-hydroxycinnamate (CINN). We also found that peroxisomes generated lactate in the presence of pyruvate. Together these data provide compelling that the Peroxisome Lactate Shuttle helps maintain organelle redox and the proper functioning of peroxisomal beta-oxidation.

Integrating metabolic pathway fluxes with gene-to-enzyme expression rates

The concept of symmorphosis emphasizes that sequential steps in physiological systems are structurally and functionally matched to each other and to in vivo maximum loads. Examination of metabolic pathways, specifically glycolysis, in this framework led to several conclusions. (i) Linked enzyme catalyzed reaction sequences are so closely integrated with each other that large changes in flux through the pathway are sustained with minimal changes in concentrations of pathway intermediates. This is true for both low and high capacity pathways and is consistent with the ‘economic design’ expectations of symmorphosis. (ii) In the glycolytic pathway, some enzymes (termed hE) occur at high concentrations and high activities, while others (lE), usually enzymes operating in vivo far from equilibrium, occur at lower concentrations and lower activities. Although genes for glycolytic enzymes are thought to be coordinately regulated by being linked to common inducing or repressing signals, during long term (phylogenetic) up or down regulation of glycolytic capacity, the expression of genes for hE type enzymes are adjusted the most; the expression of lE type (usually enzymes functioning far from equilibrium) are up or down regulated the least. These differences are of lower magnitude but are also evident in short term up or down regulation of the pathway of glycolysis (such as induction by hypoxia and repression during electrical stimulation and fiber type transformation in muscle). (iii) When considered together, these data require that, despite coordinate regulation of the overall functional unit (glycolysis), the expression pathway for each enzyme in the sequence must be under unique feedback regulation, implying an unique information flow circuit (geneienzymeigene) for each enzyme in the metabolic pathway. (iv) The matching of flux capacities in linked sequences thus seems to apply ‘across the board’—not only horizontally but also vertically—in cell metabolic design. That is, up or down change in demand for glycolytic function (horizontal pathway) is integrated with up or down regulation of gene expression (vertical pathway). While a general feedback loop from metabolism to genes has been previously recognized, the step-by-step specificity required for the pathway as a whole has been overlooked. The intriguing question of how enzymes within a single pathway self modulate or fine tune their own expression rate according to their functional role in the pathway remains unanswered, although a number of potential regulatory mechanisms are known.

Effect of high-altitude acclimation on NEFA turnover and lipid utilization during exercise in rats

Relative exercise intensity (or %maximum O2 consumption,V˙o 2 max) controls fuel selection at sea level (SL) and after high-altitude acclimation (HA) in rats. In this context we used indirect calorimetry, [1-14C]palmitate infusions, and muscle triacylglycerol (TAG) measurements to determine 1) total lipid oxidation, 2) the relationship between circulatory nonesterified fatty acid (NEFA) flux and concentration, and 3) muscle TAG depletion after exercise in HA-acclimated rats. Aerobic capacity is decreased in trained rats after 10 wk of acclimation. Both SL and HA showed the same relative use of lipids at 60% [62 ± 5% (HA) and 61 ± 3% (SL) of O2 consumption (V˙o 2)] and 80% [46 ± 6% (HA) and 47 ± 5% (SL) ofV˙o 2] of their respective V˙o 2 max. At 60% V˙o 2 max, plasma [NEFA] were higher in HA, but rate of appearance was essentially the same in both groups (at 30 min, 38 ± 9 vs. 49 ± 6 μmol ⋅ kg-1 ⋅ min-1in HA and SL, respectively). At this intensity SL showed no significant decrease in muscle TAG, but in HA it decreased by 64% in soleus and by 90% in red gastrocnemius. We conclude that 1) the relative contributions of total lipid are the same in SL and HA, contrary to differences in [NEFA], because the relationship between flux rate and [NEFA] is modified after acclimation, and 2) muscle TAG may play a more important role at HA.Relative exercise intensity (or %maximum O(2) consumption, VO(2 max)) controls fuel selection at sea level (SL) and after high-altitude acclimation (HA) in rats. In this context we used indirect calorimetry, [1-(14)C]palmitate infusions, and muscle triacylglycerol (TAG) measurements to determine 1) total lipid oxidation, 2) the relationship between circulatory nonesterified fatty acid (NEFA) flux and concentration, and 3) muscle TAG depletion after exercise in HA-acclimated rats. Aerobic capacity is decreased in trained rats after 10 wk of acclimation. Both SL and HA showed the same relative use of lipids at 60% [62 +/- 5% (HA) and 61 +/- 3% (SL) of O(2) consumption (VO(2))] and 80% [46 +/- 6% (HA) and 47 +/- 5% (SL) of VO(2)] of their respective VO(2 max). At 60% VO(2 max), plasma [NEFA] were higher in HA, but rate of appearance was essentially the same in both groups (at 30 min, 38 +/- 9 vs. 49 +/- 6 micromol. kg(-1). min(-1) in HA and SL, respectively). At this intensity SL showed no significant decrease in muscle TAG, but in HA it decreased by 64% in soleus and by 90% in red gastrocnemius. We conclude that 1) the relative contributions of total lipid are the same in SL and HA, contrary to differences in [NEFA], because the relationship between flux rate and [NEFA] is modified after acclimation, and 2) muscle TAG may play a more important role at HA.