I was particularly struck by how two papers I've recently discussed were described, so it's topical for me. One was the puzzling toxicity of a LCKD diet as published by Wang et al. This is the one using vegetable shortening of indeterminate trans fat concentration, a point sadly un-noted (or considered unimportant?) by the review. And second is the Kuwait study, described as LCKD in the review, which was not exactly glycogen depleting for a rodent.
Aside: This cited study starved rats for three days before ischaemia/reperfusion. That should have depleted glycogen AND raised raised FFAs (neither of which was checked, but any lipophobe should expect uncoupling combined with backup anaerobic glycogen reserve loss to be disastrous in ischaemia/reperfusion) as well as predictably increasing B-OHB. Combined starvation changes in fact reduce the damage produced and improve recovery. End aside.
So I'm a little ambivalent about the review and how much of the rest of their ideas I might take at face value.
Ultimately, thinking about free fatty acids, we have to talk about the control of uncoupling.
Recall this image from this study in part 29 of the Protons thread:
Free fatty acids are essential for proton transport across the inner mitochondrial membrane to uncouple oxygen consumption from ATP synthesis and to maximise electron flow down the electron transport chain with minimal resistance and minimal non essential superoxide generation.
No free fatty acids, no uncoupling. Free fatty acids are core to uncoupling.
But they are far from the only factor. For protons to be transported through the channel of the UCP by free fatty acids the channel must undergo a conformational change, which is highly dependent on the ATP status of the cytoplasm and the mitochondrial matrix.
So we have this picture from this very impressive study:
ATP in the cytoplasm fits in to a specific binding site, with each phosphate moiety of ATP fitting up against a specific arginine, all three aligning results in closure of the channel and inhibition of uncoupling, whatever the FFA concentration. Here is what the authors say:
"Moreover, residues R79 and R279 correspond to the arginines involved in nucleotide binding and protein inhibition in UCP1. According to the three-step binding model proposed for UCP1,17 β-phosphate of PN [phospho-nucleotide] binds first to R182 (helix IV, loose binding). The second step is the binding of γ-phosphate to R83 after protonation of E190 (tight binding). After the subsequent binding of α-phosphate to R276 (helix VI) the protein switches to the inhibited conformation"
Cytoplasmic ATP (and GTP) inhibit uncoupling. But not all of the time, despite the fact that there is normally always enough cytoplasmic ATP to inhibit uncoupling. So yet another factor comes in to play.
It is quite possible to inhibit the inhibition of uncoupling produced by cytoplasmic ATP.
You do this with mitochondrial ATP. ATP binding from the mitochondrial side of the channel interferes with the binding of cytoplasmic ATP but cannot reach the R83 arginine itself to close the channel. So elevated mitochondrial ATP keeps the uncoupling channel open, even in the face of rather high cytoplasmic ATP levels.
The logic to this is that if there is plenty of ATP within the mitochondria there is no need to preserve delta psi and it's fine to uncouple. If there is ATP in the cytoplasm but very little in the mitochondria the implication appears to be that ATP synthase is not generating enough mitochondrial ATP, i.e. we are either hypoxic or over-uncoupled. Continued glycolysis generates ATP on the cytoplasmic side so allows the uncoupling channel to close using this cytoplasmic ATP.
It's pretty logical.
So. Under hypoxia, whatever the level of FFAs, what happens to uncoupling?
It stops due to a lack of mitochondrial ATP. Should you fear FFAs? Only if you think you will continue to uncouple respiration under hypoxia. The balance of mitochondrial to cytoplasmic ATP should shut down uncoupling very rapidly when needed.
Just say no to Crisco (if that's how Wang et al got their result).
It has long worried me that in Veech's seminal paper on glucose, insulin and ketone metabolism in an isolated heart preparation the group was very, very careful to run the study without any involvement of free fatty acids. For those of us living in a temperate latitudes, lounging on the beach under a coconut palm while waiting for lunch to drop on our heads is not an option. Have you ever been to Lowestoft beach? No ketones without elevated FFAs at latitude 52 deg N on the North Sea coast. Fasting, or living on meat for a while, seems more likely than eating MCTs outside the tropics. I fail to see how the body would manufacture the miracle of ketones at exactly the same time as it releases the devil incarnate of free fatty acids.
Some folks like free fatty acids. Me, for one.
Some of us like uncoupling too, in the right place, at the right time.
Peter
