Eleven microorganisms, four plants, and major organs from the chicken, dog, rat, and rabbit were assayed for the presence of S-adenosylhomocysteine hydrolase, S-adenosyl-homocysteine nucleosidase, and S-ribosylhomocysteine-cleavage enzyme. All bacteria (procaryotes) were found to possess S-adenosylhomocysteine nucleosidase and S-ribosylhomocysteine-cleavage enzyme but not S-adenosylhomocysteine hydrolase. All eucaryotes tested, including yeasts, plants, birds, and mammals, possessed S-adenosylhomocysteine hydrolase but not S-adenosylhomocysteine nucleosidase or S-ribosylhomocysteine-cleavage enzyme. Of all the organs assayed in the vertebrates, the level of S-adenosylhomocysteine hydrolase was highest in liver, pancreas, and kidney, lower in spleen and testis, and very low in brain and heart. In all systems tested, equilibrium of the hydrolase reaction always favored synthesis over hydrolysis. We studied some of the kinetic properties of the hydrolase from rat liver. In the direction of synthesis, the Km value was 1.5 mM for adenosine and 4.5 mM for L-homocysteine, whereas marked substrate inhibition was observed with L-homocysteine. The condensation reaction is subject to product inhibition, and was inhibited by adenine. Results from in-vivo experiments revealed that the cells of the various organs of the dog are impermeable to the exogenously administered S-adenosylhomocysteine.
In vivo transcription of rRNA operons in Escherichia coli initiates with purine nucleoside triphosphates at the first promoter and with CTP at the second promoter.
