Rabbit liver phosphorylase a phosphatase: regulation by glucose and caffeine.

The modulation of three molecular forms of liver phosphorylase a phosphatases, the "catalytic" subunit phosphatase (35 000 relative mass (Mr) and phosphatases 2A1 and 2A2 by glucose and caffeine and some physiologically important compounds were studied, using 32P-labelled phosphorylase a obtained from rabbit liver as substrate. Glucose and caffeine showed independent and additive activations. The caffeine effect was seen at micromolar to millimolar concentrations and glucose caused activation even at concentrations below the normal blood glucose level. The nucleotides ATP and AMP, at their presumed physiological concentrations in the liver, were strongly inhibitory. Inhibition by these nucleotides and other inhibitors tested showed varied responses to the presence of the activators glucose and caffeine, depending on the phosphatase form. Thus, significant relief of ATP inhibition was afforded by glucose and caffeine acting independently for the 35 000 Mr phosphatase, whereas relief of inhibition for phosphatases 2A1 and 2A2 required a combination of glucose, caffeine, and Mg2+. The Km of the liver 35 000 Mr phosphatase was about 50 microM for the liver substrate as compared with 4 microM for the muscle substrate. The Km of phosphatase 2A2 was about 16 microM and for phosphatase 2A1 it was about 20 microM, using liver substrate in the absence of any stimulators. Mg2+ inhibited the 35 000 Mr phosphatase, but became stimulatory for phosphatase 2A2 and was an almost obligatory requirement for phosphatase 2A1.(ABSTRACT TRUNCATED AT 250 WORDS)