Thomas D. Pfister and Kenneth B. Storey
The catalytic subunit of protein phosphatase-1 (PP-1) was purified to homogeneity from final instar larvae (the overwintering stage) of freeze avoiding (Epiblema scudderiana) and freeze tolerant (Eurosta solidaginis) cold-hardy insects. Arrhenius plots showed that activity of PP-1 from both species was strongly suppressed at low temperature. Acidic shifts in pH optima and increased inhibition by okadaic acid were also observed when the enzymes were assayed at 4°C compared with 24°C. The data identify multiple ways by which PP-1 can be inhibited at low temperature and this inhibition appears to be key to sustaining high glycogen phosphorylase activity in support of polyol synthesis at low temperatures.