Fractional Rate Constants Research Articles

Synthesis and degradation of interconvertible enzymes. Kinetic equations of a model system.

Steady-state and kinetic equations have been developed which characterize the rates of formation, interconversion, and degradation of an enzyme protein subject to reversible phosphorylation. The theoretical model system incorporates separate fractional degradative rate constants for the phosphorylated and dephosphorylated protein species. The classical models for interconvertible enzymes, and for protein turnover, are special limiting situations of the general model presented here.

Protein turnover in heart and diaphragm of rat fetus.

Fractional rate constants for protein synthesis (KS) in fetal rat heart and diaphragm were determined by the constant infusion technique. Uniformly labeled [14C]tyrosine was infused at a constant rate into the maternal jugular vein of unanesthetized unrestrained pregnant rats for 6-10 h at days 19 and 21 of gestation and KS calculated from the protein-bound to tissue-free tyrosine specific radioactivity ratio and the rate constant for attainment of plateau tyrosine specific radioactivity in fetal plasma (lambda p). Rate constants for protein degradation (KD) were calculated by subtracting fractional growth rate (KG) from KS. In both tissues, protein accretion during the most rapid phase of growth (day 19) was accomplished by greater rates of synthesis and lower rates of degradation when compared with a time of lesser growth (day 21).

Triacylglycerol Secretion in Rats: Validation of a Tracer Method Employing Radioactive Glycerol

A two-compartment model was developed to analyze the temporal changes in plasma triacylglycerol (TG)-specific radioactivity after injection of [2-3H]glycerol into rats. The analysis, which yielded fractional rate constants of TG secretion, was tested in rats fed diets either adequate or deficient in essential fatty acids (EFA) and containing either glucose, fructose or sucrose as the dietary carbohydrate. The method of analysis appeared valid, first, because of a close agreement between experimental and computer-fitted TG-specific radioactivity curves, and second, because the fractional rate constants obtained were quite similar to fractional rate constants determined previously by the Triton WR-1339 technique in rats maintained on identical diets. The results show that EFA deficiency increased the fractional rate constant of TG secretion 1.7-, 1.8- and 3.3-fold and the rate of TG secretion 1.8-, 1.6- and 1.4-fold when the dietary carbohydrate was glucose, sucrose and fructose, respectively, in comparison with control rats fed diets supplying these same carbohydrates but adequate in EFA. In the latter groups, the rates of plasma TG secretion were in the range of 0.14–0.17 mg/min per 100 g body weight, and the rate of secretion in the fructose-fed rats was only 20% higher than in the glucose-fed rats.

Preservation of reactive organic matter in marine sediments

It is generally assumed that the reactivity of organic matter and the amount preserved in sedimentary deposits necessarily increases with total sedimentation rate. In some environments, such as deltas, where supply of unreactive terrigenous debris may vary independently of reactive organic matter input, the amount of reactive organic material preserved can in fact theoretically correlate either directly or inversely with sedimentation rate. The amount preserved can be shown quantitatively by transport-reaction models to depend on (1) the relative importance of electron acceptor concentration in the overlying water, (2) advection during sedimentation, (3) dilution by sedimentation, (4) solute diffusion, (5) initial flux of organic matter, and (6) the magnitudes of reaction rate constants. Both single reaction rate constant and multiple reaction rate constant models suggest that, at steady state, maximum preservation with respect to a given oxidant occurs when D s k = w 2 , where D s = whole sediment diffusion coefficient of the electron acceptor, k = first-order rate constant of the dominant organic fraction, and w = sedimentation rate. This is the likely basis for the reported correlation between average reactivity, k, of carbon in a deposit, percent carbon preservation, and w 1.5–w 2. Because of the variety of factors which determine these relationships, such correlations are probably valid only within specific classes of depositional environments.

Basic and regulatory mechanisms of in vitro release of Met-enkephalin from the dorsal zone of the rat spinal cord.

Under control conditions, superfused slices of the dorsal half of the lumbar enlargement from adult rats released Met-enkephalin-like material (MELM) that behaved as authentic Met-enkephalin under two different chromatographic procedures (Bio-gel filtration, HPLC). MELM release increased markedly on exposure of slices to batrachotoxin (0.5 microM) or to an excess of K+ (28 and 56 mM instead of 5.6 mM). The K+-evoked release was totally dependent on the presence of Ca2+ in the superfusing fluid whereas the spontaneous efflux of MELM was only partially Ca2+-dependent. Further experiments performed with tissues of polyarthritic rats indicated that the increase in their MELM levels was associated with a lower fractional rate constant of MELM release, therefore suggesting that spinal Met-enkephalin turnover might be reduced in chronically suffering animals. Examination of the possible modulation of MELM release by various neuroactive compounds present within the dorsal horn revealed that cholecystokinin (10 microM), but not its desulphated derivative, substance P-sulphoxide (10 microM), and to a lesser extent substance P, enhanced the K+-evoked MELM release. In contrast, gamma-aminobutyric acid (10 microM) and (-)-baclofen (1 microM) partially prevented the stimulatory effect of K+ on MELM release. Other compounds such as serotonin, somatostatin, and neurotensin altered neither the spontaneous nor the K+-evoked release of MELM.

Developmental aspects of sugar transport by isolated dog renal cortical tubules.

alpha-Methyl-D-glucoside (AMG) uptake was examined in isolated renal cortical tubules from newborn, 3-month-old, and adult dogs. All three age groups demonstrated active sugar transport. The initial rate of AMG uptake was similar in the 3-month-old and adult tubules which was twice that of the newborn. At steady-state, the adult and newborn tubules had achieved a similar intracellular AMG concentration which was 45% greater than that of the 3-month-old. Determination of the flux constants of these uptake patterns revealed that there was an age-dependent increase in both the net flux and the fractional influx rate constant. However, the 3-month-old had the highest fractional efflux rate constant and the newborn the lowest value with the adult in between. Kinetic analysis of AMG uptake showed a single saturable transport system for each age group. The newborn and adult had similar Km values but the 3-month-old had a value that was 60% higher. The 3-month-old tubules had the highest Vmax and the newborn tubules the lowest with the adult value in between. AMG uptake by tubules from each age group demonstrated a similar pattern of inhibition in a low sodium buffer and by glucose and phlorizin. This indicated that, aside from kinetic changes with maturation, the saturable transport system for AMG is similar in each age group.

Nervous tissue thiamine metabolism in vivo. II. Thiamine and its phosphoesters dynamics in different brain regions and sciatic nerve of the rat

Different steps of the metabolism of thiamine (T), thiamine mono- (TMP), pyro- (TPP) and triphosphate (TTP) in the cerebellum, brainstem, cerebral cortex and the sciatic nerve were evaluated in the rat in vivo. The radioactivity of T and its phosphoesters was determined at fixed time intervals (0.5–240 h) after an intraperitoneal injection of [ 14C]T (30μg:1.25 μCi), under steady state conditions. The dynamics of thiamine compounds was evaluated using a compartmental mathematical model that allowed the fractional rate constants (FRC), turnover rates (TR) and turnover times to be calculated. The phosphorylation of T to TPP and the dephosphorylations of TPP to TMP and TMP to T could be estimated in all the structures investigated. Their turnover rates were found to be ordered in the sequence: cerebellum > brainstem > cerebral cortex > sciatic nerve. The transphosphorylation of TPP to TTP was so small that it could not be determined in a reliable way. Regional differences were found both in the rate and in the composition of T and TMP mixture released from nervous structures. The shortest turnover time of TPP was found in the cerebellum, while the sciatic nerve exhibited the fastest renewal of T and TMP. In all the structures investigated TPP had a rather short turnover time, suggesting that its function might be associated to a rapid conversion into chemically different forms. The possible relationships between the rates of turnover of T compounds are the sensitivity of the nervous structures to T deficiency are discussed.

A study of calcium compartments in rat brain cortex thin slices: effects of veratridine, lithium and of a mitochondrial uncoupler.

The efflux kinetics of 45Ca from rat brain cortex thin slices previously equilibrated with it, was studied in a superfusion system. Two first order kinetic components of efflux from the tissue were found: k2 = 0.0667 min-1, that was unchanged by lowering the temperature from 37 degrees C to 15 degrees, and k3 = 0.0167 min-1 at 37 degrees C, that was reduced to 0.0897 min-1 at 15 degrees C. This suggests that k2 represents efflux from the extracellular space, and k3 that from the cellular compartment. Addition of the mitochondrial uncoupler carbonyl cyanide, m-chlorophenylhydrazone (CCCP) (10(-5)M) increased the efflux fractional rate constant of 45Ca by 35%, while no change in efflux was induced by 10 mM caffeine. Veratridine (10(-5)M) drastically reduced 45Ca efflux if superfusion was with physiological salt solution (150 mM sodium present), but not if 50 mM lithium replaced an equivalent amount of sodium in the superfusion fluid. This lithium-containing solution did not affect 45Ca efflux in the absence of veratridine. These results indicate that mitochondria accumulate only a minor fraction of intracellular 45Ca; that 45Ca possibly turns over very rapidly in the endoplasmic reticulum, and that most of 45Ca is present in a different, non-mitochondrial, non endoplasmic reticular compartment, the nature of which can be only conjectured.

Antithrombin III transfusion in patients with hepatic cirrhosis.

This study was designed to examine the effect of selective correction of antithrombin III activity on the increased turnover of fibrinogen, which occurs in patients with liver cirrhosis supposedly due to disseminated intravascular coagulation. Human antithrombin III concentrates were therefore transfused in seven patients with cirrhosis and antithrombin III deficiency (less than 80%). Fibrinogen half-life and the fractional catabolic rate constant were calculated from the turnover of 125I-fibrinogen which was represented by a two-compartment model. Prior to antithrombin III transfusion, 125I-fibrinogen half-life was 76.7 +/- 15.2 h and the fractional catabolic rate constant was 0.33 +/- 0.11 of the plasma fibrinogen pool per day. In six healthy adult controls these values were significantly different: 109.4 +/- 8.8 h and 0.19 +/- 0.01 respectively. Correction of antithrombin III activity with human antithrombin III concentrate reduced the increased turnover of radiolabelled fibrinogen to normal. The 125I-fibrinogen half-life became 108.4 +/- 17.6 h and the fractional catabolic rate constant decreased to 0.23 +/- 0.06. These observations indicate that decreased antithrombin III activity contributes in an important way to the increased 125I-fibrinogen turnover in patients with cirrhosis and this might reflect intravascular coagulation.

On the turnover of acetylcholine in mouse brain: Influence of dose size of deuterium labelled choline given as precursor

The influence of the dose size of precursor choline (Ch) on the turnover rate of acetylcholine (ACh) has been studied in whole brain and striatum of mice. Different doses of deuterium labelled Ch (d 6-Ch) were injected i.v. and concentrations of endogenous and d 6-labelled Ch and ACh were estimated at various times by mass fragmentography using deuterium labelled (d 9 − ) Ch and ACh as internal standards. At doses of 1.25–20 μmole/kg of d 6-Ch the endogenous concentration of Ch in blood wastransiently elevated for 5 min. However, the endogenous concentrations of Ch and ACh in whole brain and striatum were not affected. The fractional rate constant of ACh synthesis and efflux, K a a measure of the turnover rate of ACh, was influenced by the dose size of precursor d 6-Ch. In whole brain, K a was ca. 0.7 at doses of 5–20 μmole/kg of d 6-Ch. At lower doses K a increased to ca. 1.0. In striatum, K a was ca. 0.7 at doses of 20–40 μmole/kg of the precursor. At lower doses K a became smaller and decreased to 0.10 at a dose of 1.25 μmole/kg of d 6-Ch. When the calculations of K a were corrected for the part of d 6-Ch concentration in the brain contained in blood the figures were invariably increased by 20–40%. However, the increase was not correlated with the precursor dose size. It was found that the ratio between S ACh and S Ch decreased with increasing dose of d 6-Ch until it reached plateau values at doses of 2.5 μmole/kg and higher in whole brain and at 10–20 μmole/kg and higher in striatum. It is proposed that the decreased ratio at low concentrations of d 6-Ch depends on saturation of the high affinity (HA) uptake system for Ch coupled to ACh synthesis. At higher doses of d 6-Ch, ACh synthesis by the low affinity (LA) uptake system for Ch became increasingly important and was not yet saturated at the highest dose given (40, μmole/kg). It is suggested that the synthesis rate of ACh can be studied with either very low doses of labelled Ch that do not saturate the HA uptake system or with sufficiently high doses at which the influence of the LA uptake system on the synthesis rate of ACh is dominant.

Quantitative analysis of plasma enzyme levels based upon simultaneous determination of different enzymes

It is demonstrated that plasma elimination constants for rapidly eliminated circulating tissue enzymes can be obtained from plasma time-activity curves if a slowly eliminated reference enzyme is simultaneously sampled. Enzyme and reference enzyme must be released together into the plasma. From the elimination constants thus obtained enzyme release into the plasma can be calculated as a function of time. The method can be applied during continuous release of enzyme into the plasma. The validity of the method is tested in the dog by intravascular infusion of a preparation of cytoplasmic enzymes, obtained by incubating dog liver under anoxic conditions. Alanine aminotransferase (ALT) was used as reference enzyme. Infused quantities of aspartate aminotransferase (AST), glucosephosphate isomerase (GPI) and ALT can be estimated with coefficients of variation (CV) of respectively 10, 19 and 7.6%. Application of the method to plasma time-activity curves of enzymes in patients after acute myocardial infarction (AMI), with alpha-hydroxybutyrate dehydrogenase (HBD) as reference enzyme, results in the following values for the fractional catabolic rate constants (FCR) of AST, GPI, creatine kinase (CK) and its isoenzyme CK(MB): FCRAST = 0.093 +/- 0.006 h-1; FCRGPI = 0.27 +/ 0.03 h-1 (mean +/- SE, n = 14); FCRCK = 0.20 +/) 0.02 h-1 (n = 30); FCRCK(MB) = 0.34 +/- 0.08 h-1 (n = 16). These values are considerably higher than mentioned by most authors, and this indicates that enzyme release after AMI has been underestimated. After AMI, enzymes are released in quantities proportional to the enzyme content of human heart tissue. Average release of CK conforms to this rule but large variations for individual patients are observed. Accurate estimates of the quantities of enzymes released into the plasma can be made for slowly eliminated enzymes by the use of fixed mean values for elimination constants. The results presented to this study indicate that tissue enzymes released from infarcted myocardium in patients after AMI are recovered quantitatively in the plasma. Local inactivation of enzymes or inactivation during the transport from heart to plasma is not significant in such patients.

Catabolism of Purified Rat Fibrin(ogen) Plasmin Degradation Products in Rats

The blood clearance of purified plasmin degradation products (X, Y, D-dimer, Dcate, D EGTA, E-fibrin, Ecate and E EGTA) of rat fibrin(ogen) has been studied in a homologous system. Transcapillary flux rate constants, fractional catabolic rate constants and the ratios of intravascular and extravascular pools have been calculated. There are considerable differences in these constants between the two early fibrinogen degradation products X and Y. Also the differences between fibrinogen D and E fragments, derived from different parts of the fibrinogen molecule, are considerable. However, the differences between fibrin fragment D-dimer and fibrinogen fragment Dcate are relatively small. Also the differences between fragments E-fibrin, Ecate and E EGTA are small. The main fraction of all E-fragments is extravascular.

Measurement of effective renal plasma flow in congestive heart failure.

In the management of patients with congestive heart failure (CHF), it is often desirable to have precise knowledge of overall renal function, including the effective renal plasma flow (ERPF). It has long been recognized that ERPF is diminished in CHF. Since glomerular filtration rate is often decreased to a much lesser extent, other noninvasive procedures such as the measurement of creatinine clearances may not be entirely suitable. ERPF determination by the single plasma sampling (SPS) method affords a rapid, simple, noninvasive, and economical technique that is quite accurate and reproducible. A SPS method has been well-tested in patients following renal transplantation plus a wide variety of nephrological disorders. We have been concerned whether the SPS method would be valid in volume expanded patients. In 28 determinations of ERPF in patients with CHF, and in five patients who did not have CHF, we have found the SPS estimation of ERPF to yield results that are not clinically significantly different from those obtained by the detailed compartmental analysis method. The volumes of 131I-orthoiodohippurate (OIH) distribution were found to be somewhat higher in CHF than in controls, but fractional rate constants were proportionately lower so that intercompartmental flow rates and OIH concentrations were not different from controls. Therefore, the SPS estimation of ERPF is valid in patients with CHF and may be useful in monitoring the renal effects of various hemodynamic and pharmacological interventions.

In vivo electrochemical detection of catechols in several dopaminergic brain regions of anaesthetized rats

Differential pulse voltammetry (DPV) was used in conjunction with electrochemically treated carbon fibre microelectrodes to monitor in vivo the catechols (CA) levels in the substantia nigra (SN), ventral mesencephalic tegmentum (VMT) and nucleus accumbens of anaesthetized rats. The current corresponding to CA oxidation was always closely related to the anatomical localisation of the dopaminergic neurons e.g. it was maximal in the pars compacta of the SN. In addition, the current was selectively suppressed in all regions after 6-OHDA-induced degeneration of the dopaminergic neurons and after pargyline treatment. These findings together with our previous results suggest that 3,4-dihydroxyphenylacetic acid (DOPAC) is the main contributor to the CA oxidation current measured in these areas. Haloperidol induced a significant increase of the CA signal in the nucleus accumbens (+70%) but not in the SN or VMT. The DOPAC concentrations declined exponentially after pargyline administration. The fractional rate constant of DOPAC, computed from these results appears to be similar in all regions studied except for n. accumbens where the low rate of disappearance might be responsible for the high DOPAC concentration measured in this area. In conclusion, DPV with electrochemically treated carbon fibre electrodes is a useful means of monitoring extracellular DOPAC levels in dopaminergic brain regions.

Hippocampal distribution of acetylcholine, muscarinic receptors and cyclic GMP

The regional distribution of acetylcholine (ACh), guanosine 3′,5′-monophosphate (cyclic GMP), ACh fractional rate constant, ACh turnover rate and [ 3H]-quinuclidinyl benzilate (QNB) binding were measured in various regions of the rat hippocampus. In coronal sections of the hippocampus, concentrations of both ACh and cyclic GMP were found to be the highest in sections at the amygdaloid end. Within subsectional areas of coronal sections of the hippocampus (CA-1, CA-3, and dentate gyrus). the cyclic GMP content and the affinity of [ 3H]-QNB specific binding sites were the highest in the CA-3 and dentate gyrus regions, while the density of [ 3H]-QNB specific binding was the highest in the CA-1 area. In contrast, the ACh turnover rate was higher in the CA-3 area than in dentate gyrus or CA-1 area. This study demonstrates that an increase of ACh utilization, [ 3H]-QNB binding sites and cyclic GMP content appears to occur in some specific areas of the hippocampus. These findings suggest that the dissection described in the present experiments is useful in the study of cholinergic regulation of cyclic GMP content of some regions of the hippocampus.

Extension of the Labeled Precursor Method of Measuring Neurotransmitter Kinetics to Non‐Steady‐State Conditions: Application to Striatal Dopamine Metabolism

Abstract: A solution of the differential equations describing the single open compartment model of neurotransmitter kinetics which does not assume a steady state is presented. In addition, a computer procedure is described for analyzing data obtained following the pulse injection of radiolabeled precursors, which eliminates the use of graphical methods in calculating fractional rate constants of synthesis and catabolism. This procedure also provides statistically valid estimates of the standard errors of the rate constants, allowing for tests of the significance of experimental effects. The ability of this approach to discriminate between changes in synthesis and catabolism under non‐steadystate conditions is demonstrated by analyzing the kinetics of striatal dopamine metabolism following (a) administration of α‐methyl‐ρ‐tyrosine and (b) lesion of the substantia nigra. The utility of this method and precautions in its use are discussed.

Blood-brain barrier transport and cerebral utilization of dopa in living monkeys.

[18F]fluoro-dopa, DL-3,4-dihydroxy-5-[18F]fluorophenylalanine, a tracer for dopa, was injected intravenously into cynomolgus monkeys anesthetized with enflurane, and the appearance of 18F in the head and the disappearance of 18F from the blood were each measured continuously for 10 min with simple gamma-ray detectors. A three-compartment explanatory model was used to derive fractional rate constants for the forward flux of L-dopa into the endothelium of the brain capillaries from the blood (1.54 +/- 0.37 min-1), the back flux of dopa and/or its metabolites to the blood from the endothelium (0.67 +/- 0.05 min-1), the formation of dopamine from dopa (0.20 +/- 0.04 min-1), and the destruction of dopamine (0.04 +/- 0.02 min-1). It was also shown that only one-fifth of the dopa that entered the endothelium was available for dopamine synthesis in neurons. It was concluded that the technique described could be extended to man and used to investigate the role of dopa and dopamine in the control of mood and locomotion.

Thiamine content and turnover rates of some rat nervous regions, using labeled thiamine as a tracer

The content of total thiamine radioactivity in some nervous structures and liver of the rat was determined in a steady state condition, using [thiazole-2-14C]thiamine as a tracer. The contents were analyzed by a mamillary type compartmental model which enabled us to calculate the influx and efflux fractional rate constants, turnover times, turnover rates and relative accuracy. Total thiamine turnover rates of the central nervous system regions were found to be ordered in the following sequence: cerebellum (0.55 μg/g·h) > medulla and pons > spinal cord and hypothalamus > midbrain (plus thalamic area) and corpus striatum > cerebral cortex (0.16 μg/g·h). Sciatic nerve turnover rate was 0.58 μg/g·h. The turnover times were mainly between 5 and 10 h (range 2.4–16.4 h). The influx rate constants could be ordered as follows: cerebellum > hypothalamus, pons and medulla > corpus striatum, spinal cord, midbrain (plus thalamic area) and sciatic nerve > cerebral cortex. The results show in general a good agreement between turnover rate values and brain regional sensitivity to thiamine deficiency, the most vulnerable areas to thiamine depletion being those with the highest turnover rates.

FREE 3‐METHOXY‐4‐HYDROXYPHENYLETHYLENEGLYCOL IN THE CENTRAL NERVOUS SYSTEM OF THE RAT: SEMI‐AUTOMATED FLUOROMETRIC ASSAY, TURNOVER AND EFFECTS OF DRUGS

AbstractA semi‐automated fluorometric assay technique for free 3‐methoxy‐4‐hydroxyphenylethyl‐eneglycol (MOPEG) in rat brain and spinal cord is described.The method is based on a simple manually‐performed two‐step purification procedure using column chromatography on Sephadex G 10 and DEAE‐Sephadex A 25 (borate form) respectively. After isolation MOPEG is converted into a fluorophore in a continuous flow system using ethylene diamine condensation in the presence of an oxidant. The MOPEG assay is highly sensitive (detection limit 2 ng/sample) and linear, with an overall recovery of approx 75%.Specificity is demonstrated by testing a number of compounds and confirmed by gas chromatography‐mass spectrometry analysis. Treatment with clozapine and haloperidol (both neuroleptics), reser‐pine (impairing intraneuronal storage) or phenoxybenzamine (α‐adrenoceptor blocking agent) increased the content of MOPEG both in brain and spinal cord. Cerebral levels of MOPEG were decreased after injection of a single dose of the tricyclic antidepressant desipramine and after chronic destruction of the locus coeruleus by electrolytic lesion or by the administration of the neurotoxic drug 6‐hydroxy‐dopamine. Animals killed by microwave irradiation did not show lower MOPEG contents in brain than decapitated animals.These results indicate that MOPEG is a measure for changes in central noradrenaline turnover and that drugs affect MOPEG in the brain and spinal cord similarly. Fractional rate constants of MOPEG in the rat brain and spinal cord were estimated with the exponential decline curves produced by treatment with pargyline. Turnover rates of 193 pmol/g/h and 167 pmol/g/h in the brain and spinal cord respectively were calculated.

DT‐n‐PROPYLACETATE AND GABA DEGRADATION. PREFERENTIAL INHIBITION OF SUCCINIC SEMIALDEHYDE DEHYDROGENASE AND INDIRECT INHIBITION OF GABA‐TRANSAMINASE

AbstractThe kinetic constants for 4‐aminobutyrate: 2‐oxoglutarate aminotransferase (GABA‐trans‐aminase) and succinate‐semialdehyde: NAD+ oxidoreductase (SSA‐DH) have been determined using rat brain homogenate.The Michaelis constants for GABA‐T at saturated substrate concentrations were calculated to be Kgaba= 1.5 mM, K2‐OG= 0.25 mM, KGLU= 620 μM, and KSSA= 87 μm. The Vmax for the reaction using GABA and 2‐oxoglutarate (2‐OG) as substrates (forward reaction) was found to be 35.2 μmol/ These results indicate that MOPEG is a measure for changes in central noradrenaline turnover and that drugs affect MOPEG in the brain and spinal cord similarly. Fractional rate constants of MOPEG in the rat brain and spinal cord were estimated with the exponential decline curves produced by treatment with pargyline. Turnover rates of 193 pmol/gh and 167 pmol/g These results indicate that MOPEG is a measure for changes in central noradrenaline turnover and that drugs affect MOPEG in the brain and spinal cord similarly. Fractional rate constants of MOPEG in the rat brain and spinal cord were estimated with the exponential decline curves produced by treatment with pargyline. Turnover rates of 193 pmol/g/h and 167 pmol/g/h in the brain and spinal cord respectively were calculated.The kinetics of GABA‐T have been shown to be consistent with a Ping Pong Bi Bi mechanism. Substrate inhibition of the forward reaction, through formation of a dead‐end complex, was found to occur with 2‐OG (Ki 3.3 mM), whereas GABA was found to be a product inhibitor of the reverse reaction (Ki= 0.6 mM). Using the appropriate Haldane relationship, a Keq of 0.04 for GGBA‐T was found, indicating that the reaction was strongly biased towards GABA.For SSA‐DH, the Km of SSA was determined (9.1 μM) and the Vmax was 27.5 μmol/ These results indicate that MOPEG is a measure for changes in central noradrenaline turnover and that drugs affect MOPEG in the brain and spinal cord similarly. Fractional rate constants of MOPEG in the rat brain and spinal cord were estimated with the exponential decline curves produced by treatment with pargyline. Turnover rates of 193 pmol/g/h and 167 pmol/g These results indicate that MOPEG is a measure for changes in central noradrenaline turnover and that drugs affect MOPEG in the brain and spinal cord similarly. Fractional rate constants of MOPEG in the rat brain and spinal cord were estimated with the exponential decline curves produced by treatment with pargyline. Turnover rates of 193 pmol/g/h and 167 pmol/g/h in the brain and spinal cord respectively were calculated. h. The effect of di‐n‐propylacetate (DPA) on both GABA‐T and SSA‐DH was measured. DPA inhibited SSA‐DH competitively with respect to SSA, giving a Ki of 0.5 mM. GABA‐T was only slightly inhibited. The Ki of DPA for the forward reaction was 23.2 mM with respect to GABA, which was 40‐50 times higher than that for SSA‐DH. For the reverse reaction the Ki of DPA was found to be nearly the same (15.2 mM with respect to Glu and 22.9 mM with respect to SSA). These results suggest that GABA accumulation in the brain, after administration of DPA in vivo, is caused by SSA‐DH inhibition. Two mechanisms are indicated by the data. (1) The higher level of SSA, which results from inhibition of SSA‐DH, initiates the reverse reaction of GABA‐T, thus increasing the level of GABA via conversion of SSA. (2) The degradation of GABA is inhibited by SSA, since SSA has a strong inhibitory effect on the forward reaction, as calculated from the present data.