Copious amounts of ketones which are generated in insulin-deficient or insulin-unresponsive patients will give rise to a high anion gap metabolic acidosis.
Ketones are anions, and they form the high anion gap.
Meet the ketones
Chemically speaking, a ketone is anything with a carbonyl group between a bunch of other carbon atoms.
The above are your three typical ketoacidosis-associated ketone bodies. The biochemistry nerds among us will hasten to add that the beta-hydroxybutyrate is in fact not a ketone but a carboxylic acid, but - because it is associated with ketoacidosis, we will continue to refer to it as a ketone for the remainder of this chapter, in the spirit of convention.
In the same spirit, we can suspend our objections to acetone being included in a discussion of ketoacidosis, which (though a true ketone) is in fact not acidic or basic, as it does not ionise at physiological pH (its pKa is 20 or so).
So really, the only serious ketone acid is acetoacetate, which has a pKa of 3.77. However, beta-hydroxybutyrate is the prevalent ketone in ketoacidosis; the normal ratio of beta-hydroxybutyrate and acetoacetate is 3:1, and it can rise to 10:1 in diabetic ketoacidosis. Acetone is the least abundant.
The metabolic origin of ketones
The generation of ketones is a normal response to fasting, which follows the depletion of hepatic glycogen stores.
Let us discuss normal physiology for a change.
You, a healthy adult without serious alcohol problems, are fasting from midnight for a routine elective hernia repair. You will go to be after dinner with a few nice lumps of undigested food in your intestine, as well as about 75g of hepatic glycogen. As you sleep, you gradually digest the food and dip into the glycogen store. At the end of the night, you would wake up with something like 3-5% of your body's energy needs being met by ketones.
After a few days starvation, the ketone production ramps up, and you become increasingly reliant on them for energy. The man who has fasted for 3 days will have 30-40% of his energy needs met by ketone bodies.
So what is the origin of these ketones? With the aid of this excellent article, I will elaborate.
It all happens in the perivenous hepatocytes.
Under normal conditions, Acetyl CoA is generated by beta-oxidation of fatty acids. in the presence of ample carbohydrate fuel, there is plenty of oxaloacetate to react with acetyl-CoA, which means acetyl-CoA can easily enter Krebs cycle and generate ATP by oxidative phosphorylation, in the normal fashion. However, in a glucose-poor environment, too much oxaloacetate is diverted away into gluconeogenesis. This restricts entry into Krebs cycle for acetyl-CoA. The result is a buildup of acetyl-CoA, which is diverted into ketogenesis.
Now, on top of this, we have an increased delivery of acetyl-CoA into the process. This occurs because of the increase in beta-oxidation of fatty acids, and the increased delivery of these fatty acids into the bloodstream. This is lipolysis: a process normally inhibited by insulin, and driven by catecholamines. Imagine what may happen in a situation when the tissues are either deprived of insulin (as in Type 1 diabetes) or resistant to it (as in Type 2 diabetes). In absence of insulin, free fatty acids swarm through the circulation, exacerbating the acetyl-CoA excess.
The ratio of acetoacetate to beta-hydroxybutyrate
Acetoacetate and beta-hydroxybutyrate are freely interconverted by the enzyme beta-hydroxybutyrate dehydrogenase. All of this interconversion occurs in the mitochondria, well way from the bloodstream.
At any given time, some ratio of the two molecules exists, and this enzyme maintains that ratio.
In times of normal metabolic behaviour, that ratio is roughly 1:1.
The ratio proportions are determined by the ratio of the other participants in the reaction- specifically, the ratio of NADH to NAD+. If more NADH is available, the reaction will swing in favour of beta-hydroxybutyrate, and more acetoacetate will be converted; this is what happens in alcoholic ketoacidosis (the gentle reader is reminded that NAD+ normally outnumbers NADH by about 10 to 1, favouring the reduction of NAD+ to NADH which accompanies oxidative reactions).
The metabolic fate of ketones
Specifically, it is the metabolic fate of acetoacetate which is interesting. In my search for a good paper on this subject, I came across a series of monographs by Hans Adolf Krebs, of Krebs Cycle fame. The detail is amazing. In particular, I was awed by the drawings of the experimental apparatus, which rigorously reproduced the shape and dimensions of the glassware. But, I digress. Todays medical students are not easily persuaded by the attractive gleam of conical flasks. A more modern take on this aspect of physiology can be found here.
The metabolic fate of acetone
Before moving on to the more exciting ketone bodies, let us consider poor forgotten acetone. It, as a tenant of your bloodstream, is not utterly useless. You have several ways of evicting it, and at low concentrations much of it (up to 80%) is exhaled unchanged; the half-life seems to be about 5 hours. However, it is not its clearance, but rather its origin which is interesting: it can buffer up to a quarter of the hydrogen ion excess produced by ketoacidosis.
Acetoacetate, as an anion, decreases the strong ion difference. However, the spontaneous decomposition of the acetoacetate anion increases the strong ion difference, because it yields CO2 and acetone, both of which are inert as far as the strong ion difference is concerned. Thus, the presence of acetone is actually constructive, and helps preserve the acid-base homeostasis.
What happens to acetone itself? Numerous things. Radiolabelled carbon atoms from infused acetone were found to be lodged variably and inconsistently in the proteins and lipids of experimental patients and animals, as well as in the CO2 they exhaled. The meaning of this is uncertain, but is thought to indicate that acetone has a role to play in the hepatic regulation of acid-base balance; and acetone may also serve as a substrate for oxidative phosphorylation. However, this is far from established fact, and opinions regarding these things only ever seem to get published in journals like Medical Hypotheses.
The metabolic fate of beta-hydroxybutyrate
Boringly, beta-hydroxybutyrate is converted back into acetoacetate in the recipient cells. The mitochondria which receive this molecule clearly have a redox state which favours a more normal NADH:NAD+ ratio, which facilitates this reaction. This molecule has a plasma half-life of about 110 minutes.
The metabolic fate of acetoacetate
Ultimately, both beta-hydroxybutyrate and acetoacetate are converted back to acetyl-CoA in the recipient tissues.
This reaction does not take place in the liver, otherwise the ketone bodies would just get chewed up in the process of hepatic aerobic metabolism, and would never make it into the circulation. The reason for this seems to be a general lack of the succinyl CoA:acetoacetate CoA transferase enzyme. Thus, though the liver happily produces ketone bodies, it cannot use them for its own metabolism.
Acetoacetate is a high octane fuel. For every molecule of acetoacetate activated by coenzyme A, two molecules of acetyl-CoA are generated, which means a yield of 19 molecules of ATP (20, minus the ATP molecule required for the synthesis of one succinyl-CoA complex).
The enzymes which restore ketone bodies into Krebs cycle are most active in the heart, kidney, central nervous system and the skeletal muscle. Of these tissues, the most massive is the skeletal muscle, and it is responsible for most of the ketone metabolism, even at rest.
Urine dipstick ketone tests rely on the reaction between acetoacetate or acetone and sodium nitroprusside; this means beta-hydroxybutyrate will not be detected. Conversely, the point-of-care capillary fingerprick ketones test detects only beta-hydroxybutyrate.
When you dipstick-test somebody's urine, the ketone levels return as "+", "++" or "+++". One "+" equates to about 0.5 mmol/L of acetoacetate, whereas "+++" equates to about 3.0 mmol/L.
In a manner very similar to diabetic ketoacidosis, a lack of insulin is to blame in the case of prolonged starvation; however the difference is that the mechanisms of insulin secretion are intact, and some low baseline insulin level remains. Starvation results in decreased insulin and thus in increased lipolysis; the resulting increase in the delivery of free fatty acids to the liver exceeds the capacity of acetyl-CoA to enter the Krebs cycle, and ketogenesis results.
These are people who have just come off from a massive binge; the ketoacidosis typically strikes them on the following day.
A massive rate of lipolysis is partly to blame; this does not seem to be directly related to the alcohol, but rather to the elevated levels of cortisol and catecholamines (which suggests that your body treats a drinking binge as some sort of a critical-illness-like state.)
The redox state disturbance in the mitochondria of the alcoholic can be traced to the demands of alcohol metabolism.
The excess of alcohol requires an excess of NAD to process, in turn generating an excess of NADH; the reduced NAD+:NADH ratio favours conversion of acetoacetate into beta-hydroxybutyrate, and inhibits the reverse conversion. Ergo, in alcoholic ketoacidosis the ketone body ratio is significantly skewed in favour of beta-hydroxybutyrate. So much so, in fact, that one can use the hepatic levels of beta-hydroxybutyrate to identify alcoholics who died of ketoacidosis. One of the autopsies revealed a hepatic beta-hydroxybutyrate level of 47.2 mmol/L, which (as far as I can see) is the highest body ketone content recorded.
In diabetic ketoacidosis, beta-hydroxybutyrate is also the dominant ketone, and for similar reasons but not fto the same extent. An excellent article by Umpierez et al (2000) explores this issue in some detail, and their results are reproduced here without anypermission whatsoever. It seems that empirically, diabetic ketoacidosis patients tend to present with an elevated ratio (i.e. 3-7 times as much beta-hydroxybutyrate than acetoacetate), but still not quite as elevated as the alcoholics.
The ratio conceals the real reasons for the difference, which can confuse people Fortunately Umpierez and team were able to capture a series of ketotic patients and compare the exact concentrations of each ketone. In fact, in both diabetic and alcoholic ketoacidosis, the beta-hydroxybutyrate levels are fairly similar (as the illegally reproduced graphs clearly demonstrate). The ratio is mainly affected by the elevated acetoacetate level in diabetics. At the beginning of sampling, the average diabetic in the Umpierez study had a serum acetoacetate level of around 2.5 mmol/L, whereas the average alcoholic only had around 1.0 mmol/L (and both ended up with about 8.0 mmol/L of beta-hydroxybutyrate).
Why so much acetoacetate in diabetic ketoacidosis? The answer is probably in lipolysis, which (in the absence of insulin) becomes vastly overactive. The excess delivery of acetyl-CoA results in an excess production of acetoacetate. In the absence of alcohol there is no strong redox-related reason for all of the acetoacetate to be converted into beta-hydoxybutyrate, and so the ratio of these ketone bodies is closer to the normal 1:1 ratio. As the redox state returns to normality (with the reintroduction of insulin, or with sobriety), so the ratio of acetoacetate to beta-hydroxybutyrate seems to trend back to 1:1 - and this does not take long; within the first four hours of therapy the patients seemed to be metabolising as if their NAD/NADH ratio was back to normal.