Figure 1.

Isolated lung COX is a high-turnover isozyme. A) SDS-PAGE of COX from cow lung and liver. COX from both organs was isolated side by side, and subunits were separated by SDS-PAGE. Subunits as indicated. Lung (left lane) and liver COX (right lane) were 37% ammonium sulfate-precipitated fractions. B) Western blot analysis of isolated cow lung, liver, and heart with COX4i2 (top panel) and COX4i1 (lower panel). COX4i2 was detected in isolated lung COX. COX isolated from heart, the organ with second-highest COX4i2 transcription levels among tissues previously tested (16), gave a weak signal, whereas no signal was detected in liver COX. COX4i1 and COX4i2 run at the same size of ∼18 kDa. M, molecular marker (kDa). C) Respiration kinetics of solubilized cow lung (triangles) and liver COX (squares) in the presence of allosteric activator ADP (solid symbols), or inhibitor ATP (open symbols) and the presence of an ATP-regenerating system. COX activity (TN, turnover) is defined as consumed micromolar O₂ per second per micromolar COX. Both lung and liver COX show allosteric regulation by ADP and ATP. Lung COX is ∼2-fold more active at maximal turnover compared to liver COX in the presence of ADP or ATP. Data are representative of 3 independent experiments (sd<5% at maximal turnover).