The fitting of sequenced peptides to a high-resolution X-ray map of phosphoglycerate kinase has yielded the complete sequence and structure of the horse muscle enzyme. Metal ADP and ATP substrates are bound to one of the two widely separated domains in an environment that seems unsuitable for phosphoglycerate binding. The most plausible binding site for the phosphoglycerate substrate is on the other domain about 10 A from the ATP, which implies the possibility of a large scale hinge-bending of the domains to bring the two substrates together in a water-free environment for catalysis.

Nitric oxide (NO) synthase in rat brain was found to be constitutive and Ca2(+)-dependent. The enzyme in rat lung or liver (predominantly in parenchymal cells) was not constitutive, but was induced by endotoxin treatment and was Ca2(+)-independent. The NO synthases in rat brain and liver or lung are therefore distinct both in their properties and in their regulation.

The contribution of bicarbonate to total dissolved inorganic carbon (DIC) utilization was investigated using 18 marine phytoplankton species, including members of Bacillariophyceae, Dinophyceae, Pqmnesiophyceae, and Raphidophyceae, was assayed as an indicator of extracellular GI-catalyzed HC@ utilization. For some species, extracellular CA was constitutive, in others activity was detected under conditions of carbon limitation, and in others, even under carbon-limited conditions, activity was not detected. I n species without extracellular CA, direct HC@ uptake was investigated using a pH dnji technique in a closed system, DIC measurements, and the use of the anion exchange inhibitor 4'4'-diisothiocyanatostilbene-2,2-disulfonic acid (DLDS) . Three of these species (Chaetoceros compressus, Thalassiosira pseudonana, and Glenodinium foliaceum) gave a p H drift not inhibited by DIDS, but cultures of Chrysochromulina kappa, Gephrocapsa oceanica, and Coccolithus pelagicus, in which DLDS inhibited DIC uptake, did not gzve a pH dn$. This result shows that direct H C G transport may occur ! y a n anion exchange-type mechanism in some species but not others. Of the eighteen species investigated, only Heterosigma akashiwo did not have the potential fm direct uptake or extracellular U-catalyzed H C G utilization.Marine phytoplankton species acquire their inorganic carbon (Ci) for photosynthesis from the dissolved inorganic carbon (DIC) of the seawater.Within the usual seawater pH range of 8.0-8.3, the bulk of total DIC is HCOi, and CO, is less than 1 % of the total DIC (Skirrow 1975) when the system is in equilibrium with atmospheric CO,. Marine phytoplankton able to use HCO; may have a competitive advantage over phytoplankton using exclusively CO, when other essential nutrients or light are not rate-limiting for growth. The mechanism of DIC utilization is species dependent and some species can rapidly acclimate to changes in the concentration of dissolved CO, or total DIC (Coleman 1991, . In the direct uptake of HCO;

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