The biochemical properties of housefly flight muscle have recently been investigated to ascertain the mechanisms whereby energy is made available for the contractile processes (Sacktor, , a and b, 1954. These observations demonstrated the complete oxidation of some citric acid cycle intermediates in the sarcosomes (mitochondria) of these muscles, and showed that this oxidation could be coupled with synthesis of the energy-rich phosphate bonds in ATP. 1 Notwithstanding the multitude of enzymes in the mitochondrion, it was reported that isolated ssrcosomes were unable to oxidize glycogen and glucose (Levenbook, 1953, andSacktor, 1954). Since several experimenters had shown that these compounds are the main substances utilized during flight of flies (of. Chadwick, 1953), it appeared that extrasarcosomal enzymatic activities, too, were concerned with flight metabolism. This viewpoint was also inferred from previous results , which demonstrated a parallel in the distribution of certain enzymatic properties and the three prime morphological particulars of the flight muscle; namely, fibrils, sarcosomes, and sarcoplasm (the matrix that engulfs both fibrils and sarcosomes). Additional evidence for the localization of enzymes within these morphological entities and the relationship between them in the over-all metabolism of the flight muscle is provided in the present study.
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Fractionation ProceduresHouseflies, Musca domestic, a, of mixed sexes were used. These were reared and maintained by the procedure described previously (Sacktor, 1950).Flight muscle homogenates were prepared by grinding isolated thoraces in cold 0.9 per cent KCI with a Potter-Elvehjem homogenizer for 1 minute. The brei was then filtered through several layers of cheese-cloth to remove pieces of cuticle. The volume of the filtrate was adjusted with additional KC1 so that the final suspension contained the equivalent of 25 thoraces per ml. (10 to 15 rag. dry weight per ml.).The soluble and particulate fractions were obtained by centrifuging muscle homogenates, in the cold, for 10 minutes at about I0,000 g. The first supernatant was collected and used as 1 The following abbreviations will be used: ATP, adenosinetriphosph&te; ADP, adenosinediphosphate; AMP, adenosinemonophosphate; DPN, diphosphopyridine nucleotide; TPN, triphosphopyridine nucleotide.