Interactions between NADH-Cytochrome b5 Reductase and Cytochrome b5 Preparations Purified from Liver Microsomes*

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Abstract
Two types of NADH-cytochrome b5reductase [EC 1. 6. 2. 2] (fp1) preparations, one solubilized by treatment with detergents (d-fp1) and the other solubilized by digestion with lysosomes (1-fp1, and two types of cytochrome b5 preparations, one solubilized with detergents (d-b5) and the other by tryptic digestion (t-b5), were used to study the interactions between the flavoprotein and the cytochrome. The functional interactions were studied by measuring the NADH-cytochrome c reductase activity reconstituted from the flavoprotein and the cytochrome. Among the four possible combinations tested, the system consisting of d-fp1 and d-b5 was by far the most efficient with respect to the maximal cytochrome c-reducing activity and the cytochrome b5 concentration giving half maximal activity. Gel filtration on a Sepharose 6B column indicated that a functional aggregate of large particle size was formed on mixing d-fp1 and d-b5. However, no aggregate was detected when d-fpi was mixed with t-b5 or trypsin-treated d-b5. It is concluded that aggregate formation was principally responsible for the high catalytic activity of the system consisting of d-fp1 and d-b5

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