As described in an interesting review by Rao,1 much evidence derived from numerous experimental studies of alcoholic liver disease (ALD) support the association of endotoxemia with the initiation and progression of ALD. The review focuses on the role of endotoxemia and the mechanisms of gut barrier dysfunction in ALD. The author mentions that endotoxemia is conventionally believed to exist when the plasma endotoxin level rises higher than 2.5 endotoxin units (EU)/mL, and the endotoxin levels in ALD patients, ranging from 8.5 to 206 pg/mL, are 5- to 20-fold higher than those in normal subjects. First, I would like to comment on the determination and interpretation of endotoxemia in the clinical setting. Historically, the rabbit pyrogenic test was introduced to detect the contamination of endotoxins and other toxins in biological preparations in 1940. In the early 1960s, dose-dependent endotoxin-induced coagulation of lysates from the horseshoe crab amoebocyte was found, leading to the development of a new method of quantifying the endotoxin in 1970s, which is now called the limulus amoebocyte lysate (LAL) assay. This assay was quickly approved by the US Food and Drug Administration (FDA) because the LAL test is more sensitive, less time-consuming, more cost-effective, and better standardized than the rabbit test. For interpreting the results of the LAL assay, it should be noted that LAL is not prepared for the detection of endotoxemia but is intended only for in vitro diagnostic purposes. Endotoxin antagonists, antibiotics, plasma proteins, and unknown substances in the blood can interfere with or activate the LAL test. The term EU indicates the national standard biological activity in the LAL assay, which was determined by the FDA and other organizations on the basis of the relationship between the LAL assay and the rabbit pyrogenic test. Although the FDA initially defined 1 EU as the endotoxin activity of 200 pg of a reference standard endotoxin, the conversion ratio is variable because it is dependent on the source of the endotoxin used for each assay, ranging from 1 EU to 20 pg to 1 EU to 500 pg. Commercially available LAL kits use a control standard endotoxin because a reference standard endotoxin is very expensive. There are substantial variations in endotoxin potency among the control standard endotoxins. Additionally, substantial variation in the reactivity of lysates from different horseshoe crabs has been observed. Therefore, I would like to ask the author whether it was quite difficult to define endotoxemia and assess the differences in endotoxin levels among the studies.
The author concludes that the evidence is clear that alcohol consumption leads to increased intestinal permeability and endotoxemia, which are involved in the pathogenesis of ALD. However, there is no definite clinical evidence showing that improvements in intestinal permeability or anti-endotoxin therapy can ameliorate liver injury in ALD. Also, in an experimental study, ethanol-induced liver damage was not markedly enhanced by the long-term infusion of endotoxin to levels that were 50- to 100-fold higher than those achieved with ethanol alone.2 It is well known that a primary endotoxin challenge causes insensitivity to a secondary challenge, which is termed endotoxin tolerance, and this should be borne in mind when we try to explain the cause-effect relationship in ALD with endotoxemia.