Uniquely Inhibitable Kinases

Inhibition by a-tocopherol of protein kinase C has been reviewed in Azzi et al. (1992, 1995, 1996) and Newton (1995). Such an inhibition is not caused by a direct binding of a-tocopherol to the enzyme but by preventing its activation via phosphorylation (Tasinato et al. 1995). a-Tocopherol exerts its action independently of its free-radical scavenging capacity and most probably by interacting with a yet not characterised receptor molecule in smooth muscle cells (Boscoboinik et al. 1991, 1994, 1995). a-Tocopherol prevents uniquely protein kinase C-o phosphorylation and its functional activation. 

To date, the only MEK 1/2 kinase inhibitors to enter clinical development have shown inhibition that is not ATP competitive. This unique mechanism of inhibition of MEKl/2 has enabled compounds such as ARRY-142886 and PD0325901 to achieve excellent enzymatic and cellular selectivity for, and potency against, MEK 1/2. 

Third, we have selected aptamers that can bind to the 3II isozyme of protein kinase C from an RNA pool that spanned 120 random positions [5], The aptamers fell into several families, and individuals from two of the most prominent families were assayed for their ability to inhibit the enzymatic activity ofPKC. While these aptamers efficiently inhibited the enzymatic activity of the 3II isozyme, they had a 10-fold lower K, for the 3I isozyme (96% similar) and showed no activity against the a isozyme (80% similar). These specificities rival those seen with monoclonal antibodies. We have now selected aptamers that can bind to the a isozyme ofPKC from the same RNA pool. Sequence comparisons of the anti-pil and anti-a aptamers (Fig. 13) suggest that the map that relates target space and sequence space is convoluted. For example, while one family of aptamers was returned from both selections, other families were unique for one or the other isozyme. 

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