More on insulin and the glycerophosphate shuttle

Raphi tweeted this paper recently

Nutritional Ketosis Increases NAD+/NADH Ratio in Healthy Human Brain: An in Vivo Study by 31P-MRS

which is nice provided, as he comments, it can be replicated. There is absolutely no possible conflict of interest anywhere so long as you accept it looks like an in-house Nestlé study. I haven't knowingly bought a Nestlé product in over 30 years.

Anyway. The study looks at healthy brain biochemistry under MCT induced ketosis. The ketone oxidation (or possibly the CNS oxidation of MCTs) increases the NAD+:NADH ratio, ie moves it in the Good direction.

There is a lot of talk about the NADH generation and NAD+ depletion during glycolysis to pyruvate, shifting the ratio in the Bad direction. The assumption (with which I disagree) is that the glycerophosphate shuttle is a rescue mechanism to regenerate essential NAD+ to allow glycolysis to continue, to which I will return in a moment.

The beauty of ketones is that they do not deplete cytoplasmic NAD+ at all and only consume one mitochondrial NAD+ during the conversion of BHB to AcAc. Because this happens within the mitochondria this, plus any NADH generated at the pyruvate dehydrogenase complex, is sitting next to complex I, the most prolific re-generator of NAD+ in the cell...

All well and good and bully for ketones and the manufacturers of Peptamen®1.5 Vanilla (Nestlé Health Science SA).

This got me thinking.

Of course no one in their right mind would expect glycolysis to be arranged in such a manner as to require the glycerophosphate shuttle for simple NAD+ regeneration. This is a wasteful loss of four pumped protons and this energy will appear as heat. Think of brown adipose tissue, full of mtG3Pdh, assuming insulin is plentiful.  The correct pathway for the metabolism of glucose without insulin is to lactate without any overall depletion of cytoplasmic NAD+. Lactate can then be taken up by mitochondria exactly as ketones are. Lactate will, in the mitochondria, be reconverted to pyruvate, depleting mitochondrial NAD+ in exactly the same way as the conversion of BHB to AcAc does. Equally this happen right next door to complex I, just waiting to regenerate NAD+ and keep that NAD+:NADH ratio nice and high.

The whole point of the glycerophosphate shuttle (in Protons terms) is to facilitate insulin signalling.  Insulin is the hormone of plenty, used to encourage caloric ingress in to cells. Loss of the four pumped protons due to bypassing complex I and using mtG3Pdh instead as part of insulin signalling appears perfectly reasonable under conditions of active caloric ingress. Sustained insulin signalling causes sustained loss of cytoplasmic NADH, which generates NAD+. Once this has happened there is no longer the surfeit of cytoplasmic NADH over NAD+ from glycolysis, which is essential to drive lactate formation. Glycolysis must therefor stop at pyruvate under insulin.

Summary: For insulin signalling the glycerophosphate shuttle is active and loss of NADH requires glycolysis to abort at pyruvate.

Without insulin signalling glycolysis runs to lactate which enters mitochondria without any depletion of cytoplasmic NAD+. The lactate should enter the mitochondria, under normal physiology.


Sooooooo. This had me thinking about what would happen if, in the presence of copious glucose and copious oxygen, there was to be a sudden profound fall in absolute insulin levels. I was particularly interested in systemic lactate levels.

A sudden, profound fall in insulin levels in the presence of glucose is pathology. It generates ketoacidosis, classically from acute beta cell destruction during the onset of DMT1. There is always a profound metabolic acidosis from the failure to suppress glucagon-induced lipolysis and subsequent massive acidic ketone generation. Under the canonical view the absence of insulin should not stop NAD+ regeneration by the glycerophosphate shuttle.

What I wanted to know was whether the Protons predicted shutting down of the glycerophosphate shuttle due to hypoinsulinaemia would result in diversion past pyruvate to lactate as the end result of glycolysis. In the presence of massive levels of ketones I would also expect this lactate to appear in the systemic situation.

Does it?

Yep. Ten seconds on Google says so.

Lactic acidosis in diabetic ketoacidosis

Very nice. I had no idea this was the case because it has no direct influence on treating DKA clinically...

Peter

Of course you have to think about the chicken and egg situation with insulin and mtG3Pdh activation (I have been for years!). Which comes first? I think insulin appears to be essential, as above. I do wonder if the insulin receptor will turn out to dock with the glycerophosphate shuttle in some way...