Racemic difluoromethylornithine (D/L-DFMO) is an inhibitor of ODC (ornithine decarboxylase), the first enzyme in eukaryotic polyamine biosynthesis. D/L-DFMO is an effective anti-parasitic agent and inhibitor of mammalian cell growth and development. Purified human ODC-catalysed ornithine decarboxylation is highly stereospecific. However, both DFMO enantiomers suppressed ODC activity in a time- and concentration-dependent manner. ODC activity failed to recover after treatment with either L- or D-DFMO and dialysis to remove free inhibitor. The inhibitor dissociation constant (K(D)) values for the formation of enzyme-inhibitor complexes were 28.3+/-3.4, 1.3+/-0.3 and 2.2+/-0.4 microM respectively for D-, L- and D/L-DFMO. The differences in these K(D) values were statistically significant ( P <0.05). The inhibitor inactivation constants (K(inact)) for the irreversible step were 0.25+/-0.03, 0.15+/-0.03 and 0.15+/-0.03 min(-1) respectively for D-, L- and D/L-DFMO. These latter values were not statistically significantly different ( P >0.1). D-DFMO was a more potent inhibitor (IC50 approximately 7.5 microM) when compared with D-ornithine (IC50 approximately 1.5 mM) of ODC-catalysed L-ornithine decarboxylation. Treatment of human colon tumour-derived HCT116 cells with either L- or D-DFMO decreased the cellular polyamine contents in a concentration-dependent manner. These results show that both enantiomers of DFMO irreversibly inactivate ODC and suggest that this inactivation occurs by a common mechanism. Both enantiomers form enzyme-inhibitor complexes with ODC, but the probability of formation of these complexes is 20 times greater for L-DFMO when compared with D-DFMO. The rate of the irreversible reaction in ODC inactivation is similar for the L- and D-enantiomer. This unexpected similarity between DFMO enantiomers, in contrast with the high degree of stereospecificity of the substrate ornithine, appears to be due to the alpha-substituent of the inhibitor. The D-enantiomer may have advantages, such as decreased normal tissue toxicity, over L- or D/L-DFMO in some clinical applications.

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Barley anthers from cold pretreated spikes produced no or few calluses when plated with both loculi in contact with the medium ('flat'). When anthers were plated with only one loculus in contact with the medium ('up'), a high proportion of the anthers produced calluses. The top loculus of the 'up' anthers was most productive. 'Flat' anthers, when compared with 'up' anthers, were not only slower to produce multicellular pollen grains (MCPs) and microcalluses, but also produced fewer of them and ceased production earlier. The MCPs and microcalluses in 'flat' anthers grew more slowly and few developed beyond the 30 cell stage. These results establish the importance of anther orientation for barley anther culture.

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S U M M A R YT h e amounts of abscisic acid (ABA) in different parts of the seed have been investigated during dry storage, which induces dormancy. ABA occurs within the embryonic axis as well as in testa and cotyledon. T h e amount of ABA in the whole seed markedly declines during dry storage, although there is a transient increase within the embryonic axis. T h e incorporation of 3zP0,3into poly(A)-rich RNA within the axis declines during dry storage. Imbibition results in increased poly(A) RNA synthesis but the presence of the testa is inhibitory to this synthesis. GA, on the other hand promotes synthesis. Poly(A) RNA synthesis within isolated embryonic axes is inhibited by ABA. These results are discussed with respect to the induction, maintenance and breaking of dormancy.