Interaction of ligands in phosphorylase A as monitored by crosslinking and enzymatic modifications: synergism of glucose and caffeine manifested in the exposure of N-terminal segment.

1. Glycogen, caffeine and glucose dissociate phosphorylase a tetramer to dimers with half-maximum effect at 0.16%, 1.1 and 71 mM concentration, respectively, as monitored by crosslinking with dimethyl suberimidate at 18 degrees C. 2. The above ligands increase the rate of dephosphorylation and tryptic digestion of phosphorylase alpha at 18 degrees C in the same way with half-maximum effect at 0.04%, 0.1 and 9 mM concentration, respectively. 3. Caffeine and glucose acted synergistically in tetramer dissociation as well as in the enzymic modifications. 4. The alpha-anomer of D-glucose was twice as effective as its mutarotational equilibrium solution.