Reply to Source of acid in high-intensity exercise

[Robert Robergs, Ph.D., FASEP]

[Will here.  There has been a problem with the [EMAIL PROTECTED] 
mailbag--a long story in itself--with the result that 
<anything>@sportsci.org has come to me.  To get messages to the list, at 
the moment I have to redirect them to [EMAIL PROTECTED]  This 
glitch occurred first with this message of Rob's, but I stupidly redirected 
it to [EMAIL PROTECTED], then didn't notice that it hadn't gone to the 
list (because of the way I filter my messages to various mailbags)!  So 
here is his complete message to the list, which I quoted briefly from in my 
reply to the list.  Naturally Rob is irate!  Hope this fixes things.]

Date: Mon, 23 Jul 2001 16:24:51 -0600
From: "Robert Robergs, Ph.D., FASEP" <[EMAIL PROTECTED]>
Subject: Reply to Source of acid in high-intensity exercise
To: Will Hopkins <[EMAIL PROTECTED]>

Hi Will;

Here is my reply that I promised earlier today.

 > one molecule of glucose C6H12O6 -> 2 molecules of lactic acid C3H6O3

This is a source of confusion, as this biochemical summary is not correct.
When you summarize glycolysis and lactate production you get the following:

glucose + 2 ADP + 2 Pi + 2 NAD+ ---> 2 pyruvate + 2 NADH + 2 ATP + 2 H+ + 2
H20

2 pyruvate + 2 NADH + 2 H+ ---> 2 lactate + 2 NAD+

These summary equations are straight our of Lehninger, and have been fact
since the 1950's!

It seems that somewhere along the path of time we in exercise physiology, as
well as clinical medicine, pure physiology, and even biochemistry, have not
looked at the facts on the issue of acidosis.  Ironically, there is no
chapter on metabolic acidosis in any biochemistry text, yet as far as
exercise is concerned, it is difficult to find a more important topic!!

The biochemistry of glycolysis tells us that when starting from glucose,
lactate production is not acidifying.  To the contrary, lactate production
consumes protons.  It just so happens that when the muscle cell is
contracting to an ATP demand that exceeds the mitochodrial rate of ATP
regeneration, then additional cytosolic ATP is hydrolyzed that is not
immediately replenished by mitochondrial respiration.  Thus, the following
reaction occurs;

ATP ---> ADP + Pi + H+

When the ATP is not regenerated by the mitochondria, this free proton
contributes to acidosis.  You do not immediately see this result from
assaying concentrations of ATP, as CrP hydrolysis buffers the cellular ATP.
Consequently, this is why there is a temporal association between CrP
hydrolysis and intramuscular acidosis.  Furthermore, the decline in CrP with
increasing exercise intensity also coincides with an increase in lactate
production due to the mass action of glycolysis and enzyme kinetics of LDH
favoring lactate production under these conditions.

Given this last fact, the reason why people claim that lactate production
eventually causes acidosis is an oversimplification of the truth.  Lactate
production and acidosis are not cause and effect, just associated results of
a cell that is being overtaxed.  One could argue, based on the biochemistry,
that it is very advantageous for the cell to produce lactate to help
overcome the accumulating protons, as well of course to regenerate the NAD+.
Why have well viewed lactate production as the culprit?  My explanation is
that it is easy to do, lactate is easy to measure, and also makes the
explanation of acidosis more simplified.  In reality, the explanation of
acidosis is very complex, requires an understanding of more advanced
biochemistry, and the conditions change with changing pH due to the buffer
potential of Pi as pH lowers towards 6.8.  Unfortunately, the recent work on
the monocarbolylat lactate-H+ transporter might further reinforce the
cause-effect association/belief  between lactate production and proton
accumulation.

Finally, remember that you do not need a large increase in the concentration
of free protons to significantly lower pH from a value of 7.0.  For example,
we are talking about mmol increases in metabolites, when we are concerned
with one thousand fold less changes in protons.  Thus, acidosis does not
lend itself to stoichiometric comparisons to changes  in ATP, ADP, Pi, or
CrP.

 > Glucose or glycogen does not ionize and so does not produce hydrogen
 > ions.  Lactic acid ionizes and produces hydrogen ions.  Therefore lactic
 > acid is indeed a source of acid.   So when you go from rest (lactate ~1
mM)
 > to high-intensity exercise (lactate ~5-10 mM), you do indeed get lactic
 > acidosis.  The intermediate steps in this process are irrelevant,
 > surely?  It's a question of conservation of matter, surely?

As I have said, the intermediate steps do matter, as they show that no
protons leave lactic acid to form lactate in the LDH reaction.  The
carboxylic acid function group of 2 phosphoglycerate is not protonated when
it is formed, and remains unprotonated for the remainder of glycolysis.
Stryer provides a nice summary of glycolysis that shows where protons are
released - Hexokinase, PFK, Glyceraldehyde-3P dehydrogenase reactions.
Interestingly, the pyruvate kinase reaction consumes a proton.


 > Now, I know other things are going on.  Are those other things a source or
 > a sink of hydrogen ions?  For example, at an intensity that evokes 5-10 mM
 > lactate, phosphate will be higher, presumably from hydrolysis of creatine
 > phosphate to creatine and phosphate.  (ATP and ADP don't change
 > much.)  When you hydrolyze creatine phosphate, I presume you also get more
 > hydrogen ions, because the phosphate that was bound to creatine has picked
 > up an H (from a molecule of water; the OH goes onto the creatine, or vice
 > versa), so it will tend to ionize and release that H as H+.

No.  When you hydrolyze CrP you transfer a phosphate from ADP to ATP.  The
biochemistry of the equation is as follows;

CrP + ADP + H+ ---> ATP + Cr

The phosphate transfer of this reaction does not need water to provide an
oxygen and proton, leaving a free proton.  These events occur for ATP
hydrolysis, as a proton is required to remain on the terminal phosphate
group of ADP, thus necessitating the use of an OH- from water to use to bind
to the phosphate, which, depending on cellular pH, will ionize, releasing a
proton.  Thus, CrP hydrolysis is actually alkalinizing, and has been shown
to be such by 31P MRS.

 > There may be some mileage in considering pyruvic acid, the immediate
 > precursor to lactic acid.  Is that an even stronger acid than lactate?

Yes, but is is irrelevent, as pyruvate is already ionized!

 > Has nature hit upon lactic acid as a way to reduce acidity?  The issue is
 > complicated by the addition of two hydrogen atoms (not ions) to pyruvate
to
 > turn it into lactate: C3H4O3 + 2H -> C3H6O3.  Where those 2H come from is
 > interesting and no doubt deeply meaningful for something, but presumably
 > not relevant to the main issue that a molecule of glucose is effectively
 > isomerized into two molecules of lactic acid.

According to the organic chemistry of the reaction, one comes from NADH, and
the other is from a "free"
  proton.  Thus, lactate is a sink for protons.

 > Rob's presentation emphasized fluxes (rate of flow) of compounds through
 > biochemical steps.  I can't see the relevance of fluxes.  What matters is
 > concentrations, not fluxes.  For acidity to change, concentrations have to
 > change.

Yes, and No!  So long as ATP is regenerated by non-mitochondrial sources,
there is a net proton release that will not be detected from simply
measuring ATP and ADP.  This is why I state that flux through glycolysis and
metabolic rates are important.

Will, thanks for the stimulating query, and I hope to hear from other sports
scientists on this topic.

Rob Robergs

--------
View all messages at http://sportsci.org/forum.  Click on the links 
to JOIN, UNJOIN, alter MAIL OPTIONS, or get INFO/HELP on acceptable 
messages.