Metabolic behavior of acetyl glyceryl ether phosphorylcholine on interaction with rabbit platelets

The metabolic fate of 1-O-[³H]alkyl-2-acetyl-sn-glycero-3-phosphorylcholine ([³H]-AGEPC) upon interaction with rabbit platelets was investigated. [³H]AGEPC was converted to a product identified as the long-chain fatty acyl analog. The reaction was unaffected by extracellular calcium. After a lag time of 30 to 60 s the kinetics of the conversion was linear. The rate of the reaction was found to be a function of platelet and AGEPC concentrations. Of the [³H]AGEPC (10^?9m) 85 ± 5% was processed into the-long chain fatty acyl analog within 1 h when incubated at 37 2C with a 1.25 × 10⁹ platelets per milliliter suspension. A maximal number of 1200 to 3600 [³H]AGEPC molecules were converted to the long-chain fatty acyl derivative per minute per platelet in the presence of 2 mm EDTA. Under similar conditions the 1-O-[³H]alkyl-2-(lyso)-sn-glycero-3-phosphorylcholine ([³H]lysoGEPC) also was transformed to a comparable long-chain fatty acyl derivative at a much slower rate and to a lower extent. No significant increase in lysoGEPC was noted in incubation mixtures containing [³H]AGEPC. The possible direct transacylation of AGEPC upon interaction with platelets is discussed as well as the possible involvement of this reaction in directly triggering the platelet response to AGEPC stimuli.     

γ-Aminobutyric Acid Turnover in Rat Striatum: Effects of Glutamate and Kainic Acid

The turnover rate of gamma-aminobutyric acid (GABA) in the rat striatum was estimated by measuring its accumulation after inhibition of GABA-transaminase (GABA-T) with gabaculine. Intrastriatal injections of 100 micrograms gabaculine induced a rapid and complete inhibition of GABA-T. GABA accumulation was linear with time for at least 60 min (estimated turnover rate = 25 nmol/mg protein/h). The accumulation of GABA after gabaculine administration in animals that had been treated with kainic acid (5 nmol intrastriatally, 7 days) was only 40% of the control value, indicating that a major fraction of the net increase in GABA content induced by gabaculine originates in kainic acid-sensitive neurons. Intrastriatal injection of a mixture of kainic acid (5 nmol) and gabaculine caused a net increase in striatal GABA content significantly greater than that observed in controls, suggesting that neuronal death induced by kainic acid is preceded by a period of increased neuronal activity. Glutamic acid, the putative neurotransmitter for the excitatory corticostriatal pathway, also produced a significant increase in striatal GABA accumulation when injected together with gabaculine. This effect was blocked by the administration of the glutamate receptor antagonist glutamic acid diethyl ester. The interactions between GABAergic neurons and other neurotransmitters present in the striatum were also analyzed.     

Substrate and metal specificity in the enzymic synthesis of cyclic monoterpenes from geranyl and neryl pyrophosphate

A partially purified enzyme (carbocyclase) from the flavedo of Citrus limonum formed α-pinene, β-pinene, limonene, and γ-terpinene from geranyl pyrophosphate (GPP) and neryl pyrophosphate. The maximum specific activities obtained were 7.0 and 3.6 nmol/ min/mg, respectively. Cross-inhibition by the two substrates were observed and the ability to utilize neryl pyrophosphate was almost completely lost with aging. Citronellyl pyrophosphate and dimethylallyl pyrophosphate were the most effective inhibitors of carbocyclase. Isopentenyl pyrophosphate, the monophosphate esters of nerol and geraniol, as well as inorganic pyrophosphate were much less effective inhibitors. The enzyme had an absolute requirement for Mn²⁺. It could be replaced with about 2% effectiveness by Mg²⁺ and Co²⁺. Kinetic studies showed that the observed reaction rate correlates with the calculated concentration of the GPP (Mn²⁺)₂ species. Previous evidence with nonenzymatic reactions and the results presented support the view that the mechanism of carbocyclase may be the intramolecular analog of prenyltransferase.     

Enzymic aspects of the biosynthesis monoterpenes in plants

The biosynthesis of monoterpenes in higher plants is reviewed, with particular emphasis on recent studies of the enzymology of biosynthesis.     

Nonvitellogenic female protein in Musca domestica

During vitellogenesis in Musca domestica, a major hemolymph protein, in addition to vitellogenin, appears preferentially in females. This protein is synthesized by the adult fat bodies, secreted into the hemolymph, and is not taken up by the ovaries during vitellogenesis. We have designated this protein nonvitellogenic female protein (NVFP). It is composed of only one type of polypeptide (M_r=70,000) and occurs in two different forms. Synthesis of NVFP is induced by a protein diet, attaining maximum concentrations in females at the middle of the gonotrophic cycle. In males its maximum concentration never surpasses 10% of the concentration in females. The quantitative variation of the NVFP is cyclic and coincident with the gonotrophic cycles of Musca domestica.