Modulation of complex II-energized respiration in muscle, heart, and brown adipose mitochondria by oxaloacetate and complex I electron flow

摘要

We recently reported that membrane potential (Delta Psi) primarily determines the relationship of complex II-supported respiration by isolated skeletal muscle mitochondria to ADP concentrations. We observed that O-2 flux peaked at low ADP concentration (ADP) (high Delta Psi) before declining at higher ADP (low Delta Psi). The decline resulted from oxaloacetate (OAA) accumulation and inhibition of succinate dehydrogenase. This prompted us to question the effect of incremental ADP on respiration in interscapular brown adipose tissue (IBAT) mitochondria, wherein Delta Psi is intrinsically low because of uncoupling protein 1 (UCP1). We found that succinate-energized IBAT mitochondria, even in the absence of ADP, accumulate OAA and manifest limited respiration, similar to muscle mitochondria at high ADP. This could be prevented by guanosine 5'-diphosphate inhibition of UCP1. NAD + cycling with NADH requires complex I electron flow and is needed to form OAA. Therefore, to assess the role of electron transit, we perturbed flow using a small molecule, N1-(3-acetamidophenyl)-N2-(2-(4-methyl-2-(p-tolyl) thiazol-5-yl)ethyl)oxalamide. We observed decreased OAA, increased NADH/NAD(+), and increased succinatesupported mitochondrial respiration under conditions of low Delta Psi (IBAT) but not high Delta Psi (heart). In summary, complex II-energized respiration in IBAT mitochondria is tempered by complex I-derived OAA in a manner dependent on UCP1. These dynamics depend on electron transit in complex I.