Thursday, May 18, 2017

Fructose and metabolic syndrome: Uric acid

Some weeks ago a friend sent me a full text copy of the Naked Mole Rats (NMR) paper

Fructose-driven glycolysis supports anoxia resistance in the naked mole-rat

which demonstrated that they (NMRs) appear to generate and use fructose as a coping stratagem for dealing with hypoxia or even anoxia. This is fascinating and leads back to research in the late 1980s, mostly looking at anoxia in liver or liver cells. I'm guessing that this liver work was funded to look at ways of improving the condition of transplant grafts. Fructose is significantly better than glucose for supporting anoxic liver cells, possibly something you might expect, possibly not. Perhaps in another post.

Anyway. So I've been looking at why fructose is different to glucose and to do this you end up asking rather difficult questions about the upper sections of both glycolysis and fructolysis.

Fructose enters the fructolytic pathway by being phosphorylated very rapidly to fructose-1-phosphate. Given a large enough supply of fructose this phosphorylation can deplete the ATP supply in a cell, most obviously in hepatocytes which bear the brunt of metabolising fructose. This takes place before aldolase generates the trioses which probably (or don't, in the case of fructose) control insulin signalling through mtG3Pdh and the glycerophosphate shuttle.

If this initial ATP depletion by fructokinase is profound it is perfectly possible to take two "waste" ADP molecules and transfer a phosphate from one to the other. This generates one ATP and one AMP. The ATP is useful to the cell and the excess AMP is degraded to uric acid.

This is all basic biochemistry.

In the Protons series I have worked on the (incorrect) basis that fructose should drive the glycerophosphate shuttle hard enough to generate RET (reverse electron transport) and so signal insulin resistance. The degree of insulin resistance should neatly reduce insulin mediated glucose supply by an appropriate amount to offset the fructose and so maintain a stable flux of ATP generation from the combined fructose and glucose. That's not quite how it appears to work. Even before the aldolase step in fructolysis, the body is starting to prepare the process of insulin resistance. This paper is not unique but shows general principles:

Uric acid induces hepatic steatosis by generation of mitochondrial oxidative stress: potential role in fructose-dependent and -independent fatty liver

The title of the paper is sneaky, it doesn't give away the answer! Nor does the abstract. If you don't want to read the paper, the missing link is NOX4.

NADPH oxidase 4 (NOX4), if exposed to uric acid (present here from fructolysis induced AMP degradation), translocates to the mitochondria and starts to generate enough hydrogen peroxide* to down regulate aconitase, abort the TCA and divert citrate out of the mitochondria through the citrate/malate shuttle for DNL. This will not just affect fructose metabolism, acetyl-CoA from glucose, entering the TCA as citrate, will also be diverted to DNL.

*The NOX family appear to be the only enzymes with no function other than to produce ROS, mostly superoxide. NOX4 is unique in that it always produces hydrogen peroxide. There is uncertainty if the "E loop" of the enzyme converts superoxide to hydrogen peroxide directly or if this is a docking site for superoxide dismutase, which does the conversion as an accessory module to NOX4.

I think it is a reasonable assumption that the hydrogen peroxide generated by NOX4 will be what signals the insulin resistance induced by fructose, rather than RET via mtG3Pdh. Quite why fructose doesn't drive the glycerophosphate shuttle is a difficult question to answer. Obviously the aldolase products of fructose-1-P (fructolysis) differ from those of fructose-1-6-bisphosphate (glycolysis) but these pathways are very difficult to get at experimentally and I've not found any papers looking at what controls why dihydroxyacetone phosphate from fructolysis doesn't drive mtG3Pdh, but that appears to be the case. There are hints that some activation of the glycerophosphate shuttle does occur but NOX4 seems to be the main player. It might relate to the consumption of NADH in the conversion of glyceradehyde to glycerol and so reducing the need to decrease it using the glycerophosphate shuttle. Hard to be sure.

So. Uric acid is the evil molecular link between fructose and metabolic syndrome via NOX4. And yes, yes, you can block metabolic syndrome using allopurinol to reduce uric acid production in rats but you have to give them a sh*tload of it. After that NOX4 might be considered evil or hydrogen peroxide is evil or aconitase is evil when it's on strike. Lots of drug targets available for molecular cleansing.

My own concept is that there is the necessity to developing insulin resistance when fructose is available so as to limit glucose ingress to offset the ATP from that fructose ingress. If that is done by NOX4, so be it. The facility to deal with fructose by the generation of hydrogen peroxide is not random, it's not some accidental mistake perpetrated by evolution on hapless humans who munched on a few Crab apples or found a little honey. It is an appropriate evolutionarily response to a relatively common occurrence. The fact that uric acid mediated insulin resistance is common to alcohol metabolism as well as to fructose metabolism suggests that this mechanism is a general approach to dealing with a calorie input which takes priority over metabolising glucose.

Developing a drug along the lines of allopurinol to block uric acid production, or an inhibitor of NOX4, or a hydrogen peroxide scavenger to avoid insulin resistance is simply trying to block a perfectly adaptive response to a reasonable dose of fructose.

All that's needed to avoid a pathological response to fructose is to avoid ingesting a pathological dose of the stuff. There is actually quite a lot of evidence to suggest that physiological levels of uric acid production might be beneficial...


Mulkidjanian: Na+ pump or Na+/H+ antiporter?

Mulkidjanian is a co-worker with Skulachev and extremely wedded to the primacy of Na+ bioenergetics, which is good. He has been looking at NuoH and NuoN subunits of complex I and their phylogenetics. In contrast to this, in the past I've discussed similarities between NuoH and NuoL. You just have to accept we're never going to be certain which component of complex I is most closely related to another... Anyway, I like this paper:

Phylogenomic Analysis of Type 1 NADH:Quinone Oxidoreductase

"Two recently published works independently noted the structural similarity between the NuoH and NuoN subunits and suggested their origin by some ancestral membrane protein duplication [13, 14]. Our analysis does not exclude the possibility that this duplication may have occurred even before the LUCA stage. In this case the initial NDH-1 form [proto-Ech in my terminology] had only one type of membrane subunit (the ancestor of NuoN and NuoH), which could function as a sodium transporter. The duplication of the gene would result in a different subunit, which improved the kinetic effectiveness of the redox-dependent sodium export pump (that participated in maintenance of [K+]/[Na+] greater than 1 in a primal cell) by facilitating proton translocation in the reverse direction".

Bear in mind that none of us can be certain exactly what a given protein might have been doing based on these family trees of genes.

I think there is general agreement that ancestor of NuoH and NuoN is a membrane pore and that it is primordial. In Mulkidjanian's scenario that pore is associated with a redox driven hydrogenase. His idea is that the hydrogenase is using preformed ferredoxin, or something similar, to extrude Na+ ions from the cell. This requires an external source of energy and his concept is for ZnS catalysed photosynthesis giving a localised organic "soup", ie heterotrophy. The refs are here and here. The source of K+ for the primordial cell cytoplasm is suggested here. I have to say, I'm not a convert to these aspects of his ideas, I'm staying more aligned with autotrophic thinking...

My own view is that the pore was a duct to localise oceanic acidic pH tightly to an NiFeS hydrogenase within alkaline vent "cytoplasm" to allow the hydrogenase to reduce ferredoxin, the primary energy currency of the proto-cell. The power source is the pH differential across an internal FeNiS moiety within the hydrogenase, combined with molecular hydrogen as the electron donor to reduce ferredoxin and so, eventually, CO2.

Given the almost certain ancestral gene duplication it is not difficult to make an antiporter out of NuoH/NuoN, whether you consider the ancestor to have been a proton pore or part of a Na+ pump. Even today, the membrane component of Complex I functions as an antiporter for Na+/H+ provided you separate it off from the hydrophilic matrix section:

The deactive form of respiratory complex I from mammalian mitochondria is a Na+/H+ antiporter

Given an antiporter sitting in a Na+ opaque membrane we can antiport a ton of Na+ out of the cell using a geological proton gradient to give us the result of a low intracellular Na+ concentration. Excess Na+ extrusion can be converted, by electrophoresis, to an elevated K+ inside giving the modern intracellular composition. In the early days the electrophoresis might not have been K+ specific, theoretically any positive ion other than Na+ would do. K+ is the long term preferred option.

As soon as we leave the vent there is no free antiporting so we need to have a system which provides energy to generate a Na+ potential (buffered by K+ electrophoresis). The power available to do this becomes very limited in the absence of a geothermal proton gradient, when all that is available is the reduction of CO2 using H2, the Wood–Ljungdahl pathway. The Na+ chemiosmotic circuit then comes in to it's own as a system for combining small amounts of free energy in to units large enough to generate one ATP molecule. Recall how the modern pyrophosphatase Na+ pump requires the hydrolysis of four PPi to give one ATP via chemiosmotic addition. Until the advent of photosynthesis and the possibility of heterotrophy, all free living prokaryotes would have been autotrophic and living on a meagre energy budget.

The switch from luxurious hydrothermal vent conditions to lean autotrophic conditions goes a long way to explaining the universality of chemiosmosis. Alkaline hydrothermal vents may be stable on geological time scales but not for 4 billion years of un-interrupted flow and if the Wood–Ljungdahl pathway is all there is to replace the vent power supply it's going to be chemiosmosis all the way...