Barbiturate-induced α-amylase synthesis in barley endosperm

Abstract

Phenobarbital has been found to promote the synthesis and secretion of α-amylase by embryoless barley seeds. Optimal activity was observed at a phenobarbital concentration of 1.0 mM, which promoted 39–82% (average, 57%) as much α-amylase formation as did saturating concentrations of gibberellic acid (GA3). Barley half-seeds incubated with 0.1 mM phenobarbital secreted as much protein as did those treated with 1.0 μM GA3. The kinetics of release of α-amylase from half-seeds incubated with either phenobarbital or GA3 appeared identical. Phenobarbital likely induces a de novo synthesis of α-amylase since the increase in enzyme activity was almost totally blocked by cycloheximide and 6-methylpurine. Besides phenobarbital, only cyclobarbital, amytal, hexobarbital, and thiopental showed activity among a large number of barbiturates and related drugs which were tested. In the phenobarbital molecule, the carbonyl oxygen at position 2 and the hydrogen on the nitrogen at position 3 of the barbituric acid ring are absolute requirements for activity since both 2-desoxyphenobarbital and methylphenobarbital elicited no response. Substitution of the phenyl moiety of phenobarbital with any group other than a cyclohexenyl (cyclobarbital) or an isoamyl (amytal) gave complete inactivity. Some possible mechanisms for the mode of action of phenobarbital in the barley endosperm system are discussed with particular reference to what is currently known regarding the inductive action of this barbiturate in mammalian liver.