Endoplasmic reticulum, membrane kinetics

Of particular interest from the physiological point of view is the fact that the Km value for glucose, as defined by this kinetic treatment [see Eq. (5b)] [which equals Ki for glucose in Eqs. (8a) and (9a) since k., = k, and fc i = fc8], contains a water concentration term in the numerator. Conceivably, in the environment of the enzyme within the phospholipid-rich membrane of the endoplasmic reticulum, water concentrations may be much less than 55.5 M, and the Km value for glucose under such conditions would be appreciably lower (10). 

The P450 enzymes are found primarily in the outer membrane of the endoplasmic reticulum. Enzyme activity requires that the enzyme be integrated into a membrane that contains P450 reductase and, for some reactions, cytochrome b5. Characterization of the saturation kinetics for the P450 enzymes can be determined using a variety of enzyme preparations, including tissue slices, whole cells, microsomes, and reconstituted, purified enzymes. The more intact the in vitro preparation, the more it is likely that the environment of the enzyme will represent the in vivo environment. However, intact cell preparations do not... 

The C5(6) desaturase is a membrane bound enzyme localized in the endoplasmic reticulum, and for the first time, we have developped an enzymatic bioassay. The enzymatic A -sterol fomied was detected using its typical UV absorbance at 281,5 nm. The properties of the microsomal system have been studied and the kinetics of the desaturation reaction has been established [1]. 

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