Inhibition of PP5 by okadaic acid. The purified catalytic subunit of PP5 was assayed, using [32P]labeled phosphohistone as a substrate as described in Materials and Methods. The data is expressed as % of controls, with control activity being 4.2 ± 0.25 μmole/min/mg protein. Okadaic acid was mixed with the enzymes for 10 min at 23°C prior to the initiation of the reaction with the addition of substrate. Inhibition assays contained ~ 200 pM PP5. The data represent the mean SD (n = 4).