Robert L. Tansik

Synthesis of prostaglandin 6-keto F1α by cultured aortic smooth muscle cells and stimulation of its formation in a coupled system with platelet lysates

Lysed aortic smooth muscle cells, when incubated with [14C] arachidonate, synthesized only one radioactive product, which was identified as 6-keto-PGF1alpha. Formation of this product from smooth muscle cell lysates was stimulated when human platelet extracts were added to the system, and further stimulation was observed when imidazole, a selective inhibitor of thromboxane synthesis, was added to this coupled system. These observations indicate that the cyclooxygenase of the smooth muscle cells was rate-limiting, that the prostacyclin synthetase of these cells can utilize endoperoxides produced by platelets, and that blocking of thromboxane synthesis might, under certain conditions, shunt arachidonate metabolism toward prostacyclin formation.

X-Ray Crystallographic Studies of Candida Albicans Dihydrofolate Reductase. High Resolution Structures of the Holoenzyme and an Inhibited Ternary Complex.

The recent rise in systemic fungal infections has created a need for the development of new antifungal agents. As part of an effort to provide therapeutically effective inhibitors of fungal dihydrofolate reductase (DHFR), we have cloned, expressed, purified, crystallized, and determined the three-dimensional structure ofCandida albicans DHFR. The 192-residue enzyme, which was expressed in Escherichia coli and purified by methotrexate affinity and cation exchange chromatography, was 27% identical to human DHFR. Crystals of C. albicans DHFR were grown as the holoenzyme complex and as a ternary complex containing a pyrroloquinazoline inhibitor. Both complexes crystallized with two molecules in the asymmetric unit in space group P21. The final structures had R-factors of 0.199 at 1.85-Å resolution and 0.155 at 1.60-Å resolution, respectively. The enzyme fold was similar to that of bacterial and vertebrate DHFR, and the binding of a nonselective diaminopyrroloquinazoline inhibitor and the interactions of NADPH with protein were typical of ligand binding to other DHFRs. However, the width of the active site cleft of C. albicans DHFR was significantly larger than that of the human enzyme, providing a basis for the design of potentially selective inhibitors.

Effects of Δ9-tetrahydrocannabinol and cannabidiol on phospholipase and other enzymes regulating arachidonate metabolism

delta 9-Tetrahydrocannabinol (THC) and cannabidiol (CBD) caused a marked stimulation of phospholipase A2 when incubated with intact human platelets that were prelabeled with [14C] arachidonate. CBD was about 1.5 x as potent as THC in the same concentration range (10 leads to 80 microM) Most of the released arachidonate was converted to lipoxygenae products. When [14C] arachidonate was incubated with lysed platelet extracts, THC inhibited both thromboxane synthetase and prostaglandin cyclooxygenase, so that the net effect was a redistribution of products toward the lipoxygenase pathway at the same time that a decrease in total cyclooxygenase product formation occurred. THC did not directly affect arachidonate lipoxygenase. Both TCH and CBD also stimulated release from prelabeled neuroblastoma cells (NBA2), which do not contain an active lipoxygenase pathway. In this case, accumulation of free arachidonate was detected by autoradiography. The multiple effects of THC and CBD on phospholipase A2 and arachidonate metabolism may mediate some of the pharmacological actions of these compounds, such as their anticonvulsant, anti-inflammatory, and hypotensive properties.

Prostaglandin synthesis in homogenates of cultured neuroblastoma cells

Homogenates of 5 neuroblastoma cell lines found to produce prostaglandin products from exogenous [14C]arachidonate, with specific enzyme activities ranging from 60 to 365 pmol per min per mg protein. Under identical conditions a glial cell line was much less active. PGF2 alpha and PGE2 were the major products from neuroblastoma cells, with PGF2 alpha predominating in all cases. The prostaglandin synthesizing activity of neuroblastoma extracts was at least an order of magnitude higher than activities reported for endogenous prostaglandin synthesis in brain tissues. The pattern of products was similar to that achieved after incubation of a rat brain microsomal extract with [14C]arachidonate, although the enzyme activity of neuroblastoma was about 200-fold higher. The presence of a relatively high prostaglandin cyclooxygenate activity in cultured neuroblastoma cells is of particular interest in that these cells may be useful model systems for studies of some aspects of neuronal prostaglandin synthesis.

Preclinical development of eniluracil: Enhancing the therapeutic index and dosing convenience of 5-fluorouracil

Eniluracil (5-ethynyluracil, GW 776, 776C85) isbeing developed as a novel modulator of 5-fluorouracil (5-FU) forthe treatment of cancer. Eniluracil is an effective mechanism-based inactivator of dihydropyrimidine dehydrogenase (DPD), thefirst enzyme in the catabolic pathway of 5-FU. By temporarilyeliminating this prevalent enzyme, eniluracil providespredictable dosing of 5-FU and enables oral administration of5-FU to replace intravenous bolus and continuously infuseddosing. New DPD is synthesized with a half-life of 2.6 days. Italso eliminates the formation of problematic 5-FU catabolites.Most importantly, in laboratory animals, eniluracil increases thetherapeutic index and absolute efficacy of 5-FU. Accompanyingreports in this journal indicate that eniluracil has promisingclinical potential.