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

We recently reported that membrane potential (Δψ) primarily determines the relationship of complex II‐supported respiration by isolated skeletal muscle mitochondria to ADP concentrations. We observed that O2 flux peaked at low ADP concentration ([ADP]) (high Δψ) before declining at higher [ADP] (low Δψ). 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 Δψ 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 succinate‐supported mitochondrial respiration under conditions of low Δψ (IBAT) but not high Δψ (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.—Fink, B. D., Yu, L., Sivitz, W. I. Modulation of complex II‐energized respiration in muscle, heart, and brown adipose mitochondria by oxaloacetate and complex I electron flow. FASEB J. 33, 11696‐11705 (2019). www.fasebj.org