Cytoplasmic fractions of rat myometrium. 1. General description and some enzymic properties

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Abstract
Rat uterine muscle from pregnant animals was homogenized in 0.3 [image] -sucrose and fractionated by differential centrifuging to yield an easily sedimentable fraction, N, composed of cell nuclei, myofilaments and connective tissue; a mitochondrial fraction, M; a microsomal fraction, P; and a clear final supernatant. Fraction M was contaminated to the extent of about 25% with fragments of collagen fibers. It could oxidize substrates of the tricarboxylic acid cycle, but was incapable of oxidative phosphorylation. In the absence of metal activators the adenosine triphosphatase (ATPase) activity of fraction M was about l0[mu] moles of phosphate/hr./mg N; it was not increased by "aging" but was stimulated about fourfold by 0.1 m[image]-dinitrophenol (DNP). Mg++ produced a 10- to 20-fold stimulation of ATPase. DNP stimulation was abolished by aging, but that due to Mg++ was only slightly diminished. Fraction P was composed of fragments of the sarcoplasmic reticulum (vesicles, many of which had attached dense bodies). It possessed an ATPase activity greater than that of M; this activity was almost completely dependent on bivalent cations, and was practically unaffected by DNP. The choice of medium for centrifugal fractionation is discussed.