Measuring Transient Reaction Rates from Nonstationary Catalysts

Up to now, methods for measuring rates of reactions on catalysts required long measurement times involving signal averaging over many experiments. This imposed a requirement that the catalyst return to its original state at the end of each experiment—a complete reversibility requirement. For real catalysts, fulfilling the reversibility requirement is often impossible—catalysts under reaction conditions may change their chemical composition and structure as they become activated or while they are being poisoned through use. It is therefore desirable to develop high-speed methods where transient rates can be quickly measured while catalysts are changing. In this work, we present velocity-resolved kinetics using high-repetition-rate pulsed laser ionization and high-speed ion imaging detection. The reaction is initiated by a single molecular beam pulse incident at the surface, and the product formation rate is observed by a sequence of pulses produced by a high-repetition-rate laser. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the principle of this approach by rate measurements on two simple reactions: CO desorption from and CO oxidation on the 332 facet of Pd. This approach overcomes the time-consuming scanning of the delay between CO and laser pulses needed in past experiments and delivers a data acquisition rate that is 10–1000 times higher. We are able to record kinetic traces of CO₂ formation while a CO beam titrates oxygen atoms from an O-saturated surface. This approach also allows measurements of reaction rates under diffusion-controlled conditions.

High aerobic exercise capacity predicts increased mitochondrial response to exercise training

 

Low exercise capacity is a strong predictor of all-cause cardiovascular mortality and morbidity. In contrast, high exercise capacity is protective and “physical fitness” is considered beneficial. These effects seem to be mediated through mitochondrial function. Importantly, exercise capacity consists of an intrinsic (genetic) and an extrinsic (exercise, environmental) part. In humans, these two parts cannot be truly separated. The rat model of high (HCR) and low (LCR) capacity runners allows to distinguish between the two parts. We assessed mitochondrial function in this model, specifically investigating the impact of exercise training on mitochondrial respiratory capacity.

HCR and LCR were divided into control and exercised groups. Exercise capacity was determined individually using a ramped test. Animals were trained five times a week for four weeks on a treadmill. Mitochondria were isolated from heart, M. gastrocnemius and liver. Citrate synthase activity and protein content were determined photometrically and respiratory capacity was measured using a Clark-type electrode.

At the same age and tibia length, LCR-C were heavier and had a lower heart to body weight ratio than HCR-C. Citrate synthase activity was lower in skeletal muscle of LCR but cardiac citrate synthase was not different between sedentary HCR and LCR. Respiratory capacity in heart and liver was not different between sedentary HCR and LCR but was lower in skeletal muscle in LCR compared to HCR with all selected substrates (glutamate: 86,0±17,6 vs. 63,7±8,0; succinate: 203±19 vs. 136±17 nAO/min/mg Protein). Exercise training led to an increase in body weight in HCR but did not change body weight in LCR. Similarly, gastrocnemius and soleus weights only increased with exercise in HCR. Exercise led to an increase in citrate synthase activity in hearts of HCR (0,78±0,07 vs. 1,58±0,45 U/mg Protein) but not of LCR. Consistently, mitochondrial respiratory capacity was found increased in HCR with exercise in heart with all substrates (glutamate: 261±43 vs. 305±35; succinate 417±32 vs. 539±65 nAO/min/mg Protein). Liver was not affected by exercise.

Conclusion

Our data suggest that genetic predisposition for aerobic capacity additionally affects the response of mitochondria to exercise. Thus, it may be possible that the “born runner” benefits more from aerobic exercise training than the “less genetically equipped counterpart”.

Funding Acknowledgement

Type of funding source: None

High-fat diet affects skeletal muscle mitochondria comparable to pressure overload-induced heart failure

In heart failure, high-fat diet (HFD) may exert beneficial effects on cardiac mitochondria and contractility. Skeletal muscle mitochondrial dysfunction in heart failure is associated with myopathy. However, it is not clear if HFD affects skeletal muscle mitochondria in heart failure as well. To induce heart failure, we used pressure overload (PO) in rats fed normal chow or HFD. Interfibrillar mitochondria (IFM) and subsarcolemmal mitochondria (SSM) from gastrocnemius were isolated and functionally characterized. With PO heart failure, maximal respiratory capacity was impaired in IFM but increased in SSM of gastrocnemius. Unexpectedly, HFD affected mitochondria comparably to PO. In combination, PO and HFD showed additive effects on mitochondrial subpopulations which were reflected by isolated complex activities. While PO impaired diastolic as well as systolic cardiac function and increased glucose tolerance, HFD did not affect cardiac function but decreased glucose tolerance. We conclude that HFD and PO heart failure have comparable effects leading to more severe impairment of IFM. Glucose tolerance seems not causally related to skeletal muscle mitochondrial dysfunction. The additive effects of HFD and PO may suggest accelerated skeletal muscle mitochondrial dysfunction when heart failure is accompanied with a diet containing high fat.

Cytosolic, but not matrix, calcium is essential for adjustment of mitochondrial pyruvate supply

Mitochondrial oxidative phosphorylation (OXPHOS) and cellular workload are tightly balanced by the key cellular regulator, calcium (Ca²⁺). Current models assume that cytosolic Ca²⁺ regulates workload and that mitochondrial Ca²⁺ uptake precedes activation of matrix dehydrogenases, thereby matching OXPHOS substrate supply to ATP demand. Surprisingly, knockout (KO) of the mitochondrial Ca²⁺ uniporter (MCU) in mice results in only minimal phenotypic changes and does not alter OXPHOS. This implies that adaptive activation of mitochondrial dehydrogenases by intramitochondrial Ca²⁺ cannot be the exclusive mechanism for OXPHOS control. We hypothesized that cytosolic Ca²⁺, but not mitochondrial matrix Ca²⁺, may adapt OXPHOS to workload by adjusting the rate of pyruvate supply from the cytosol to the mitochondria. Here, we studied the role of malate-aspartate shuttle (MAS)-dependent substrate supply in OXPHOS responses to changing Ca²⁺ concentrations in isolated brain and heart mitochondria, synaptosomes, fibroblasts, and thymocytes from WT and MCU KO mice and the isolated working rat heart. Our results indicate that extramitochondrial Ca²⁺ controls up to 85% of maximal pyruvate-driven OXPHOS rates, mediated by the activity of the complete MAS, and that intramitochondrial Ca²⁺ accounts for the remaining 15%. Of note, the complete MAS, as applied here, included besides its classical NADH oxidation reaction the generation of cytosolic pyruvate. Part of this largely neglected mechanism has previously been described as the “mitochondrial gas pedal.” Its implementation into OXPHOS control models integrates seemingly contradictory results and warrants a critical reappraisal of metabolic control mechanisms in health and disease.

Vibrational Relaxation Lifetime of a Physisorbed Molecule at a Metal Surface

Previous measurements of vibrational relaxation lifetimes for molecules adsorbed at metal surfaces yielded values of 1-3 ps; however, only chemisorbed molecules have been studied. We report the first measurements of the vibrational relaxation lifetime of a molecule physisorbed to a metal surface. For CO(v=1) adsorbed on Au(111) at 35 K the vibrational lifetime of the excited stretching mode is 49±3  ps. The long lifetime seen here is likely to be a general feature of physisorption, which involves weaker electronic coupling between the adsorbate and the solid due to bonding at larger distances.