Mitochondrial cytochrome c oxidase is inhibited by ATP only at very high ATP/ADP ratios.

In the past, divergent results have been reported based on different methods and conditions used for enzymatic activity measurements of cytochrome c oxidase (CytOx). Here, we analyze in detail and show comparable and reproducible polarographic activity measurements of ATP-dependent inhibition of CytOx kinetics in intact and non-intact rat heart mitochondria and mitoplasts. We found that this mechanism is always present in isolated rat heart mitochondria and mitoplasts; however, it is measurable only at high ATP/ADP ratios using optimal protein concentrations. In the kinetics assay, measurement of this mechanism is independent of presence or absence of Tween-20 and the composition of measuring buffer. Furthermore, the effect of atractyloside on intact rat heart mitochondria confirms that (i) ATP inhibition occurs under uncoupled conditions [in the presence of carbonly cyanide m-chlorophenyl hydrazone (CCCP)] when the classical respiratory control is absent and (ii) high ATP/ADP ratios in the matrix as well as in the cytosolic space are required for full ATP inhibition of CytOx. Additionally, ATP inhibition measured in intact mitochondria extends in the presence of oligomycin, thus indicating further that the problem to measure the inhibitory effect of ATP on CytOx is apparently due to the lack of very high ATP/ADP ratios in isolated mitochondria.

Overlapping Autoimmune Diseases: A Case Report and Review of Eosinophilic Granulomatosis With Polyangiitis and Mixed Connective Tissue Disease.

We describe a rare case of concurrent eosinophilic granulomatosis with polyangiitis and mixed connective tissue disease in a 27-year-old man who presented with pulmonary, renal, cardiac, and skin manifestations. We confirmed the diagnosis based on clinical, histopathological, and serological criteria. We treated the patient with corticosteroids, methotrexate, cyclophosphamide, and hydroxychloroquine, achieving early remission. The coexistence of both conditions in the same patient is extremely rare and has only been reported in a few cases worldwide. We also review the literature on these two rare autoimmune diseases' coexistence, pathogenesis, diagnosis, and management. Our case emphasizes recognizing overlapping autoimmune conditions in patients with complex clinical features and employing a comprehensive diagnostic approach and tailored treatment strategies. Further research is needed to understand these patients' epidemiology, prognosis, and optimal therapy. Early diagnosis and aggressive immunosuppression are crucial for achieving remission and preventing organ damage. We also identified the knowledge gaps and research needs in this field.