Interaction of Adrenodoxin with P4501A1 and Its Truncated Form P450MT2 through Different Domains: Differential Modulation of Enzyme Activities

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Abstract
Recently we showed that the β-naphthoflavone-inducible liver mitochondrial P450MT2 consists of two N-terminal truncated forms of the microsomal P4501A1, termed P450MT2a (+5/1A1) and MT2b (+33/1A1) [Addya et al. (1997) J. CellBiol. 139, 589−599]. In the present study, we demonstrate that intact P4501A1 and the major mitochondrial form, P450MT2b (routinely referred to as P450MT2), show distinct substrate specificities and preference for different electron transport proteins. Enzyme reconstitution and spectral studies show that the wild-type adrenodoxin (Adx), but not the mutant Adx, binds to P450MT2 in a functionally productive manner (Kd = 0.6 μM) and induces a characteristic high-spin state. Adx binding to intact P4501A1 or +5/1A1 is less efficient as seen from spectral shift patterns (Kd = 1.8−2.0 μM) and reconstitution of enzyme activity. Use of Adx−Sepharose affinity matrix yielded <90% pure P450MT2 (specific activity: 13.5 nmol/mg of protein) starting from a partially purified fraction of 10−15% purity, further demonstrating the specificity of P450MT2 and Adx interaction. Chemical cross-linking studies show that the bovine Adx forms heteroduplexes with both P450MT2 and intact P4501A1, though at different efficiencies. Our results show that Adx interacts with P450MT2 through its C-terminal acidic domain 2, while interaction with intact P4501A1 likely involves the N-terminal acidic domain 1. These results point to an interesting possibility that different electron transfer proteins may differently modulate the enzyme activity. Our results also demonstrate for the first time as to how a different mode of Adx interaction differently modulates the substrate specificities of the two P450 forms.