Monanu M O, Madsen N B
Biochem Cell Biol. 1987 Apr;65(4):293-301. doi: 10.1139/o87-038.
The natural substrate (phosphorylase a) and two alternative ones (phosphorylated histone and a tetradecapeptide consisting of residues 5-18 of rabbit skeletal muscle phosphorylase a) were used to distinguish the modes of action of some physiologically important effectors of four different molecular forms of rabbit liver phosphorlase a phosphatases. In general, glucose, caffeine, AMP, ADP, Pi, and glucose-1-P showed substrate-directed effects for the holophosphatase forms, since they usually did not affect the activity on histone phosphate and, with one slight exception (Pi), never affected the activity on the tetradecapeptide phosphate. ADP, Pi, and glucose-1-P did affect directly the relative mass (Mr) 35,000 phosphatase, in addition to an inhibition mediated via phosphorylase a. ATP exerted both substrate- and enzyme-directed effects for the Mr 35,000 phosphatase and phosphatases 1 and 2A2, but only a substrate-directed effect for phosphatase 2A1, suggesting that the gamma-subunit of the type 2 phosphatases may prevent ATP binding to the phosphatase. Mg2+ showed substrate-directed effects for phosphatases 1, 2A1, and 2A2, and an additional enzyme-directed effect for the Mr 35,000 phosphatase form. Furthermore, Mg2+ could not abolish ATP inhibition of the tetradecapeptide phosphatase activity, but significantly overcame ATP inhibition of the phosphorylase a phosphatase activity, thus suggesting that its ability to reverse the ATP effect is by a substrate-directed mechanism. The substrate-directed effects seen for the different ligands on the different phosphatase forms strongly indicate the significance of this form of control in the regulation of phosphorylase a phosphatase activities and may serve to narrow the otherwise broad substrate specificities of the major phosphorylase a phosphatase activities in mammalian tissues: phosphatases 1 and 2A.
使用天然底物（磷酸化酶a）以及另外两种替代底物（磷酸化组蛋白和由兔骨骼肌磷酸化酶a的5 - 18位残基组成的十四肽）来区分兔肝磷酸化酶a磷酸酶四种不同分子形式的一些生理重要效应器的作用模式。一般来说，葡萄糖、咖啡因、AMP、ADP、Pi和葡萄糖 - 1 - P对全磷酸酶形式表现出底物导向效应，因为它们通常不影响对磷酸化组蛋白的活性，并且除了一个轻微例外（Pi），从不影响对磷酸化十四肽的活性。ADP、Pi和葡萄糖 - 1 - P除了通过磷酸化酶a介导抑制作用外，还直接影响相对分子质量（Mr）35,000的磷酸酶。ATP对Mr 35,000磷酸酶以及磷酸酶1和2A2表现出底物导向和酶导向效应，但对磷酸酶2A1仅表现出底物导向效应，这表明2型磷酸酶的γ亚基可能会阻止ATP与磷酸酶结合。Mg2 +对磷酸酶1、2A1和2A2表现出底物导向效应，对Mr 35,000磷酸酶形式还表现出额外的酶导向效应。此外，Mg2 +不能消除ATP对十四肽磷酸酶活性的抑制作用，但能显著克服ATP对磷酸化酶a磷酸酶活性的抑制作用，因此表明其逆转ATP效应的能力是通过底物导向机制实现的。不同配体对不同磷酸酶形式所观察到的底物导向效应强烈表明这种控制形式在调节磷酸化酶a磷酸酶活性中的重要性，并且可能有助于缩小哺乳动物组织中主要磷酸化酶a磷酸酶活性（磷酸酶1和2A）原本宽泛的底物特异性。
