New Insights into the Search for Life on Mars

The discovery by the Lander Phoenix (summer 2008) that the Mars polar soil is rich of perchloric acid salts (Na, Mg, Ca perchlorate) strongly could change the interpretation of the Martian experiment of 14CO2 release (LR, Labeled release experiment), performed in 70’s by both Viking Landers. The LR experiment gave substantially positive results but, at that time, possibility of Martian bacteria was ruled out because the CGMS instruments on board of both Vikings didn’t detect any trace of complex organic molecules. But Martian organics exist and were found in fair quantities by Curiosity, landed inside the Gale crater on 2012. So it is likely that Viking CGMS, working at about 500°C, could not see any organic substances (natural or bacterial) because, at that temperature, perchlorates decompose, releasing Oxygen that destroyed organics BEFORE their detection. In any case, the discovery of keragenic compounds by Curiosity, could also be indication of a presence of archea bacteria in the distant past of Mars, when the atmosphere of the Red Planet was wetter and denser than now.

The effect of respiratory chain impairment of β-oxidation in rat heart mitochondria

Cardiac ischaemia leads to an inhibition of β-oxidation flux and an accumulation of acyl-CoA and acyl-carnitine esters in the myocardium. However, there remains some uncertainty as to which esters accumulate during cardiac ischaemia and therefore the site of inhibition of β-oxidation [Moore, Radloff, Hull and Sweely (1980) Am. J. Physiol. 239, H257-H265; Latipää (1989) J. Mol. Cell. Cardiol. 21, 765–771]. When β-oxidation of hexadecanoyl-CoA in state III rat heart mitochondria was inhibited by titration of complex III activity, flux measured as 14CO2 release, acid-soluble radioactivity or as acetyl-carnitine was progressively decreased. Low concentrations of myxothiazol caused reduction of the ubiquinone pool whereas the NAD+/NADH redox state was less responsive. Measurement of the CoA and carnitine esters generated under these conditions showed that there was a progressive decrease in the amounts of chain-shortened saturated acyl esters with increasing amounts of myxothiazol. The concentrations of 3-hydroxyacyl and 2-enoyl esters, however, were increased between 0 and 0.2 µM myxothiazol but were lowered at higher myxothiazol concentrations. More hexadecanoyl-CoA and hexadecanoyl-carnitine were present with increasing concentrations of myxothiazol. We conclude that 3-hydroxyacyl-CoA dehydrogenase and acyl-CoA dehydrogenase activities are inhibited by reduction of the ubiquinone pool, and that this explains the confusion over which esters of CoA and carnitine accumulate during cardiac ischaemia. Furthermore these studies demonstrate that the site of the control exerted by the respiratory chain over β-oxidation is shifted depending on the extent of the inhibition of the respiratory chain.

Metabolism of alpha-ketoisocaproic acid in isolated perfused liver of cirrhotic rats

To determine the hepatic fate of alpha-ketoisocaproate (KIC) in cirrhosis, six groups of isolated rat livers were perfused with 0, 0.5, 1 (with or without alpha-[1-14C]KIC), 2, and 5 mM KIC; control livers from healthy rats were studied in parallel under similar conditions. KIC was rapidly removed by the normal livers, whereas uptake was lower in the cirrhotic livers at all concentrations tested (at 2 mM, 4.04 +/- 0.33 vs. 6.32 +/- 0.58 mumol/min; P < or = 0.05). The transamination pathway, evaluated by leucine exchanges, was more important in the cirrhotic livers (25.4 vs. 6.8% in controls at 2 mM). The incorporation of alpha-[1-14C]KIC in proteins of cirrhotic liver was increased compared with controls (0.25 +/- 0.04% of alpha-[1-14C]KIC was incorporated in proteins excreted in perfusate vs. 0.20 +/- 0.04 in controls; P < or = 0.05). In addition, a line of evidence suggests that glutamine rather than glutamate is the N donor for leucine synthesis from KIC. The decarboxylation pathway evaluated by beta-hydroxybutyrate production and by 14CO2 release from alpha-[1-14C]KIC was reduced, respectively, by 40-85% (according to KIC dose) and by 24% at 90 min in cirrhotic livers compared with healthy livers. These results indicate a dramatic modification of KIC metabolism in the cirrhotic liver; its uptake by the liver is decreased and its incorporation into proteins is increased via an enhancement of transamination to leucine, probably as a consequence of an inhibition of branched-chain keto acid dehydrogenase.

The oxidation of leucine in tumour-bearing rats

Rats bearing the Walker-256 carcinosarcoma showed significant changes in leucine metabolism compared with their non-tumour-bearing controls. After a single intravenous tracer dose of L-[1-14C]leucine in vivo, 14CO2 release by tumour-bearing rats was significantly elevated throughout the time course of administration. In addition, both the clearance and turnover rates of the tracer were significantly enhanced in these animals. Incubation of soleus muscles from control and tumour-bearing rats in the presence of L-[1-14C]leucine revealed an enhanced oxidation of the amino acid in the tumour-bearing group. Tumour tissue slices were also able to oxidize the tracer at a similar rate to that found in soleus muscles from control animals.

Skeletal muscle mitochondrial β-oxidation. A study of the products of oxidation of [U-14C]hexadecanoate by h.p.l.c. using continuous on-line radiochemical detection

Well-coupled mitochondrial fractions were prepared from rat skeletal muscle without the use of proteolytic enzymes. The products of [U-14C]hexadecanoate oxidation by rat skeletal muscle mitochondrial fractions were analysed by h.p.l.c. with on-line radiochemical detection. In the presence of 1 mM-carnitine, 70% of the products is acetylcarnitine. In agreement with Veerkamp et al. [Veerkamp, van Moerkerk, Glatz, Zuurveld, Jacobs & Wagenmakers (1986) Biochem. Med. Metab. Biol. 35, 248-259] 14CO2 release is shown to be an unreliable estimate of flux through beta-oxidation in skeletal muscle mitochondrial fractions. The flux through beta-oxidation is recorded unambiguously polarographically in the presence of 1 mM-carnitine and the absence of citrate cycle intermediates.

Effects of variation of the tri-iodothyronine/reverse triiodothyronine ratio in vivo on the metabolic characteristics of subsequently isolated hepatocytes

1. Reverse tri-iodothyronine (3,3′,5′-tri-iodothyronine, rT3), a major product of the peripheral monodeiodination of thyroxine, was administered subcutaneously to fed rats at a dose of 100 μg/100 g body weight for 2 consecutive days. 2. This dose induced a 17-fold increase in plasma rT3 (from 0.05±sem 0.01 to 0.85±0.11 ng/ml, P > 0.001) whilst the plasma T3 concentration was decreased to half of the control value (0.40 ± 0.03 to 0.20 ± 0.02 ng/ml, P > 0.01). 3. As a result of these changes the T3/rT3 ratio was therefore decreased from 8.0 ±1.6 to 0.23 ± 0.03 (P > 0.001). 4. Hepatocytes prepared from control or rT3treated rats were incubated with [l-14C]oleate and the rates of 14CO2 release and glucose production were estimated. Despite the changes in ratio of T3 to rT3 observed in vivo, rates of 14CO2 release and glucose production rate from hepatocytes subsequently isolated were unchanged.

Role of hexose monophosphate shunt in parathyroid hormone secretion

The metabolism of labeled glucose by collagenase-dispersed bovine parathyroid cells was examined. When the medium calcium ion concentration was increased to 2.0 mM, the rate of 14CO2 release from [1-14C]glucose was increased 169 +/- 45% compared with the rate of 0.5 mM calcium. There was no significant change in the rate of 14CO2 release from [6-14C]glucose by this maneuver. The greatest increase in 14CO2 release and decrease in parathyroid hormone secretion occurred between medium calcium ion concentrations of 0.5-1.5 mM. This difference in the metabolism of glucose represents a true increase in hexose shunt activity because the incorporation of label from either [1-14C]- or [6-14C]glucose into parathyroid tissue lipids was equal. This suggests equilibration of label at the level of triose-phosphates. The increase in hexose shunt activity was not due to a calcium-mediated increase in glucose uptake because calcium changes did not affect 2-[3H]deoxyglucose transport by the cells. Phenazine methosulfate added to cells incubated at 0.5 mM calcium selectively increased hexose shunt activity in a dose-dependent manner (91 +/- 33% overall) and concomitantly inhibited parathyroid hormone secretion 65% overall at 0.5 mM calcium. The compound 6-aminonicotinamide inhibited hexose shunt activity but could not overcome the inhibition of hormone secretion at 2.0 mM calcium. A decrease in protein biosynthesis cannot fully explain the inhibition of hormone secretion by calcium or phenazine methosulfate because [3H]-leucine incorporation into total cell protein was not as affected as secretion.(ABSTRACT TRUNCATED AT 250 WORDS)