Conversion of metmyoglobin directly to oxymyoglobin by mitochondria from pork muscle (M. masseter) and liver

摘要

The colour of meat is determined by the amount and state of myoglobin. The balance between electron transfer to reduce met-myoglobin at the outer mitochondrial membrane and the reduction of oxygen to water at complex IV, cytochrome c oxidase, is not well understood at present Many phenomena take place post mortem that could change the present theories of metmyoglobin reduction. A better understanding of the robustness of mitochondrial complexes and the mitochondrion's structural and in particular outer membrane intactness post mortem are needed. We have studied mitochondrial structural changes through sedimentation experiments and through oxygen respiration protocols. Liver and muscle mitochondria metabolizing on succinate in the presence of rotenone with horse metmyoglobin added were studied. Cytochrome c was added to mitochondria and permeabilized cells respiring on succinate after blocking complex I with rotenone at different pH values. Oxygen consumption was measured by high-resolution respirometry (Oroboros Oxygraph-2K instrument). After oxygen depletion (oxygen concentration close to zero) reduction of met-myoglobin with concomitant binding of oxygen was observed as increased absorbance at 544 and 582 nm. The reduction time seemed to reflect oxygen consumption rate of the respiratory chain and not the degree of outer membrane intactness. We suggest that many cells in muscle and liver have defect outer mitochondrial membranes post mortem, and the outer mitochondrial cytochrome b_5 might therefore not strictly be involved in met-myoglobin reduction. The sedimentation coefficient of mitochondria from pork liver in isotonic medium vary with pH between 10-20.000 S; the larger values at low pH indicated structural modifications