Mitochondrial membranes physical properties

In addition, the in situ localization of RPH activity within the liver cell still remains to be defined. It was suggested (Prystowsky et al., 1981) that the unusual subcellular distribution observed might have resulted in part from the hydrophobic physical properties of the enzyme, which led it to become absorbed to particulate material (i.e., membranous components) that sediments in the nuclear (and mitochondrial-lysosomal) fraction. Within the cell, these hydro-phobic properties may well direct the enzyme to be associated with retinyl ester-containing lipid droplets or lipid-protein aggregates (discussed further below). Thus, the hydrophobic properties of the enzyme may be physiologically involved in enabling it to perform its role of catalyzing lipid ester hydrolysis. 

Paclobutrazol treatment of Agraveolens and Rgracilis resulted in changes in the phospholipid and sterol compositions of both the plasma and mitochondrial membranes. These changes led to alterations in the physical properties of these membranes.The results suggest that sterol production and sterol type may in some way influence phospholipid biosynthesis. 

The oligomycin-sensitive ATPase complex, a major enzyme of the mitochondrial inner membrane, can serve to illustrate some of these principles. The ATPase complex is a water-insoluble enzyme which spontaneously associates into vesicular membranes in the presence of phospholipids. The ultrastructural appearance of the reconstituted ATPase membranes is similar to that of the native inner mitochondrial membrane. The complex consists of at least 10 different subunit polypeptides which have been resolved into 3 components, each with a measurable function (1) Five of the polypeptides are part of a catalytic unit called Fi, a water-soluble ATPase which neither requires phospholipids for activity nor has any detectable capacity for binding phospholipids. (2) Four other polypeptides form a unit which has no presently known enzymatic function, but when combined with Fi modifies its physical and catalytic properties. The proteins of this unit are extremely insoluble in water and can combine with phospholipids to form membranes. One of the proteins of the membrane unit is characterized by an unusually large proportion of nonpolar amino acids and a high affinity for phospholipids. (3) The third component (OSCP) is a single polypeptide whose function is to link F to the hydrophobic membrane unit. In the purified state, this protein is completely water soluble. ... 

A second possibility to account for the observed defect in DNA synthesis during growth at low unsaturated acid levels is that the decreased fluidity of the membrane under these conditions directly affects the process of mitochondrial DNA replication. It has been suggested that mitochondrial DNA synthesis is a membrane-associated process. Recent work in our laboratory has demonstrated that the fluidity of the mitochondrial membrane has a direct influence on mitochondrial DNA replication in vitro An Arrhenius plot of DNA replication in vitro has a discontinuity (Fig. 5), the temperature of which varies with the unsaturated fatty acid content of the mitochondrial membranes (Table II). These results demonstrate the existence of a close functional relationship between mitochondrial DNA replication and the physical properties of the mitochondrial membrane. 

Whereas phosphorylation efficiency dropped during ADP limitation under normoxia, P 0 ratios were conservatively five-fold higher under severe hypoxia, when oxygen consumption was limited by oxygen supply. A reduction in proton leakage across the inner membrane under hypoxia could potentially explain the observation, which would decrease state 4 respiration. The proton permeability of the inner mitochondrial membrane is dependent on the physical properties of... 

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