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The glycogen phosphorylase of Tetrahymena pyriformis

1971; Elsevier BV; Volume: 143; Issue: 1 Linguagem: Inglês

10.1016/0003-9861(71)90188-3

1096-0384

Varda Kahn, J.J. Blum,

Hemoglobin structure and function

Some kinetic properties of an AMP-independent glycogen phosphorylase (EC 2.4.1.1) of Tetrahymena pyriformis have been examined. The Km value for glucose-1-phosphate was about 2.4 mm and was unaffected by AMP. Glucose-6-phosphate did not affect the activity of the enzyme, but UDPG and ATP inhibited, the former being more potent than the latter. When added to reaction mixtures, EDTA inhibited the activity of the enzyme very rapidly. This inhibition could not be reversed by the addition of divalent ions or by the removal of EDTA by dialysis or gel filtration. Preincubation of the enzyme with EDTA or ATP caused an irreversible inactivation. The rate of inactivation was slower than the rate of inhibition mentioned above. ATP was much less effective than EDTA in inactivating the enzyme. The inactivation caused by ATP was proportional to concentration and to time of exposure to ATP but the kinetics of EDTA inactivation were more complex. AMP and structurally related nucleoside monophosphates neither stimulated nor inhibited the activity of Tetrahymena phosphorylase but they could, however, protect the enzyme against the inhibiting effects of ATP and EDTA. There was a good correlation between the ability of nucleoside monophosphates to protect against ATP inactivation and against EDTA inhibition. During storage, however, the sensitivity of the enzyme to EDTA inactivation changed differently from its sensitivity to ATP inactivation, indicating that EDTA and ATP probably inactivated the enzyme by different mechanisms. The relative concentrations of UDPG, ATP, and AMP may play an important regulatory role in vivo. An additional regulatory mechanism, depending on the growth conditions of the organism, is possible since it was shown that phosphorylase isolated from glucose-grown cells was much more resistant to heat inactivation and to inhibition by EDTA and by ATP than the enzyme isolated from cells grown in the absence of glucose.