The organic nuclei of chlorophylls, haems, cytochromes and vitamin B12 are biosynthesised from a single tetrapyrrolic intermediate which has an unexpected, rearranged structure. The mechanism of biosynthesis of this key intermediate has now been characterised in detail. Some of the information thereby obtained is also of use in the investigation of human diseases such as the porphyrias.

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When the enzyme deaminase acts alone on porphobilinogen, it releases a transient intermediate into the medium which is unaffected by further treatment with a large excess of deaminase. The intermediate undergoes rapid ringclosure chemically (4 ca. 4 min) to form uraporphyrinogen-I. '3C Spectroscopic studies on the intermediate generated from 13C labelled porphobilinogen combined with synthesis of labelled standards for determination of chemical shifts establish its structure to be a linear tetrapyrrole, the unrearranged hydroxymethylbilane. Other workers deduced a different, cyclic structure (preuro'gen) which is shown here to be incorrect by chemical studies, l3C spectroscopy and 13C :15N double-labelling experiments. That the intermediate is the unrearranged hydroxymethylbilane is confirmed by its unambiguous synthesis. The natural and synthetic samples of this bilane are shown to be excellent and identical substrates for cosynthetase (free from deaminase) with production of uroporphyrinogen-Ill. Thus, deaminase is the enzyme for assembly of four porphobilinogen units to the linear tetrapyrrole stage and cosynthetase is the ring-closing and rearranging enzyme. Two proposals are discussed for the mechanism of inversion of the terminal ring-D of the hydroxymethylbilane in the formation of uroporphyrinogen- Ill.THE formation of uroporphyrinogen-111 (uro'gen-111) (3) and ammonia from four molecules of porphobilinogen (PBG) (1) is catalysed in living systems by two proteins, deaminase and cosynthetase. Cosynthetase is easily destroyed by heat-treatment and deaminase alone acts on PBG (1) to produce uroporphyrinogen-I (uro'gen-I) * This proposal grew as a result of a valuable discussion with Dr. D. C. Williams (Trinity College, Dublin) who we warmly thank, on the possible involvement of methylene-tetrahydrofolate in porphyrin biosynthesis.

Two routes are explored for the synthesis of chlorins geminally substituted in the reduced ring; both involve cyclisations of 1 -pyrroline (3,4-dihydropyrrole) systems promoted by copper( 11) salts. In the preferred synthesis, a pyrrolomethyl-1 -pyrroline is combined with a 5-bromo-5'-bromomethylpyrromethene; though not high yielding, this approach involves few steps from readily prepared building blocks.

The fluoroethoxy and fluoropropoxy substituted 2-(6-chloro-2-phenyl)imidazo[1,2- a]pyridin-3-yl)- N, N-diethylacetamides 8 (PBR102) and 12 (PBR111) and 2-phenyl-5,7-dimethylpyrazolo[1,5- a]pyrimidin-3-yl)- N, N-diethylacetamides 15 (PBR099) and 18 (PBR146) were synthesized and found to have high in vitro affinity and selectivity for the peripheral benzodiazepine receptors (PBRs) when compared with the central benzodiazepine receptors (CBRs). The corresponding radiolabeled compounds [ (18)F] 8 [ (18)F] 12, [ (18)F] 15, and [ (18)F] 18 were prepared from their p-toluenesulfonyl precursors in 50-85% radiochemical yield. In biodistribution studies in rats, the distribution of radioactivity of the [ (18)F]PBR compounds paralleled the known localization of PBRs. In the olfactory bulbs, where the uptake of radioactivity was higher than in the rest of the brain, PK11195 and Ro 5-4864 were able to significantly inhibit [ (18)F] 12, while little or no pharmacological action of these established PBR drugs were observed on the uptake of [ (18)F] 8, [ (18)F] 15, and [ (18)F] 18 compared to control animals. Hence, [ (18)F] 12 appeared to be the best candidate for evaluation as an imaging agent for PBR expression in neurodegenerative disorders.