PubMed · 4405741
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G Brichard. 1972. [Practolol in anesthesia].. https://pubmed.ncbi.nlm.nih.gov/4405741/
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To probe the mechanistic origins of convex Eyring plots that have been observed for alpha-chymotrypsin (alpha-CT)-catalyzed hydrolysis of specific p-nitroanilide substrates [Case, A., and Stein, R. L. (2003) Biochemistry 42, 3335-3348], we determined the temperature-dependence of (15)N-kinetic isotope effects for the alpha-CT-catalyzed hydrolysis of N-succinyl-Phe p-nitroanilide (Suc-Phe-pNA). To provide an interpretational context for these enzymatic isotope effects, we also determined 15N-KIE for alkaline hydrolysis of p-nitroacetanilide. In 0.002 and 2 N hydroxide (30 degrees C), 15N-KIE values are 1.035 and 0.995 (+/-0.001), respectively, and are consistent with the reported [HO-]-dependent change in rate-limiting step from leaving group departure from an anionic tetrahedral intermediate in dilute base, to hydroxide attack in concentrated base. For the alpha-CT-catalyzed hydrolysis of Suc-Phe-pNA, 15N-KIE is on kc/Km and thus reflects structural features of transition states for all reaction steps up to and including acylation of the active site serine. The isotope effect at 35 degrees C is 1.014 (+/-0.001) and suggests that in the transition state for this reaction, departure of leaving group from the tetrahedral intermediate is well advanced. Significantly, 15N-KIE does not vary over the temperature range 5-45 degrees C. This result eliminates one of the competing hypotheses for the convex Eyring plot observed for this reaction, that is, a temperature-dependent change in rate-limiting step within the chemical manifold of acylation, but supports a mechanism in which an isomerization of enzyme conformation is coupled to active site chemistry. We finally suggest that the near absolute temperature-independence of 15N-KIE may point to a unique transition state for this process.
CONTEXT: Many patients with chronic angina experience anginal episodes despite revascularization and antianginal medications. In a previous trial, antianginal monotherapy with ranolazine, a drug believed to partially inhibit fatty acid oxidation, increased treadmill exercise performance; however, its long-term efficacy and safety have not been studied in combination with beta-blockers or calcium antagonists in a large patient population with severe chronic angina. OBJECTIVES: To determine whether, at trough levels, ranolazine improves the total exercise time of patients who have symptoms of chronic angina and who experience angina and ischemia at low workloads despite taking standard doses of atenolol, amlodipine, or diltiazem and to determine times to angina onset and to electrocardiographic evidence of myocardial ischemia, effect on angina attacks and nitroglycerin use, and effect on long-term survival in an open-label observational study extension. DESIGN, SETTING, AND PATIENTS: A randomized, 3-group parallel, double-blind, placebo-controlled trial of 823 eligible adults with symptomatic chronic angina who were randomly assigned to receive placebo or 1 of 2 doses of ranolazine. Patients treated at the 118 participating ambulatory outpatient settings in several countries were enrolled in the Combination Assessment of Ranolazine In Stable Angina (CARISA) trial from July 1999 to August 2001 and followed up through October 31, 2002. INTERVENTION: Patients received twice-daily placebo or 750 mg or 1000 mg of ranolazine. Treadmill exercise 12 hours (trough) and 4 hours (peak) after dosing was assessed after 2, 6 (trough only), and 12 weeks of treatment. MAIN OUTCOME MEASURES: Change in exercise duration, time to onset of angina, time to onset of ischemia, nitroglycerin use, and number of angina attacks. RESULTS: Trough exercise duration increased by 115.6 seconds from baseline in both ranolazine groups (pooled) vs 91.7 seconds in the placebo group (P =.01). The times to angina and to electrocardiographic ischemia also increased in the ranolazine groups, at peak more than at trough. The increases did not depend on changes in blood pressure, heart rate, or background antianginal therapy and persisted throughout 12 weeks. Ranolazine reduced angina attacks and nitroglycerin use by about 1 per week vs placebo (P<.02). Survival of 750 patients taking ranolazine during the CARISA trial or its associated long-term open-label study was 98.4% in the first year and 95.9% in the second year. CONCLUSION: Twice-daily doses of ranolazine increased exercise capacity and provided additional antianginal relief to symptomatic patients with severe chronic angina taking standard doses of atenolol, amlodipine, or diltiazem, without evident adverse, long-term survival consequences over 1 to 2 years of therapy.
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