Alcoholic beverages are commonly consumed by adults and food worldwide and are enjoyed because alcohol stimulates our appetite and makes meals more delicious. However, excessive intake increases the risk of various diseases. Vegetable and fruit consumption reduces the risk of alcohol-induced gastric cancer, suggesting that food components may affect the absorption and metabolism of ethanol. Therefore, it would be significant to determine food components that prevent alcohol-related diseases. In the present study, we have shown that shiitake extract suppresses elevation in plasma ethanol concentration. The suppressive effect was higher in the Shiitake-H group than in the Shiitake-N group, indicating that lentinic acid, a flavor precursor, would be one of the functional components in shiitake.

The lentinic-acid content in Shiitake-H and Shiitake-N was first determined to ensure that Shiitake-H contained lentinic acid and Shiitake-N did not. This difference in lentinic-acid content in Shiitake-H and Shiitake-N indicates that γ-glutamyl transpeptidase and C-S lyase were inactivated by methanol during the preparation of Shiitake-H, and these enzymes converted lentinic acid to volatile cyclic sulfur compounds during the trial of Shiitake-N.

The Shiitake-H group being administered shiitake extract containing 300 µmol lentinic acid in 10 mL of 20% ethanol/kg body weight showed lower plasma ethanol elevation from 30 to 180 min after oral administration than the control group. We have previously reported oral administration of a garlic extract with 500 μmol S-Allyl-L-cysteine sulfoxide (ACSO) in 10 mL of 20% ethanol/kg body weight (Garlic-H group) significantly suppressed plasma ethanol elevation 30 to 360 min after oral administration. These results indicate that 300 µmol lentinic acid/kg body weight is similar to 500 μmol ACSO/kg.

Two mechanisms can be considered for the suppressive effect of Shiitake-H on the elevation of ethanol concentration in the plasma: (1) suppression of ethanol absorption from the gut, (2) promotion of ethanol metabolism in the stomach and liver. We carried out experiments to determine which of these mechanisms is responsible for this effect of Shiitake-H. As about 10–30% ethanol is absorbed from the stomach and the rest is mainly from the jejunum, we injected ethanol together with Shiitake-H into the ligated loop of the stomach or jejunum, and ethanol concentration in the portal vein was measured.

As a result, ethanol concentration in the portal vein was significantly lower in the Shiitake-H group than in the control and Shiitake-N groups from 5 to 60 min after the injection. Moreover, residual ethanol concentration in the stomach was significantly higher in the Shiitake-H group than in the control and Shiitake-N groups. As no transporters are used for the absorption of ethanol in the gut, the suppressive effect of Shiitake-H on ethanol absorption might be attributable to the retardation of peristaltic movement.

Further study is necessary to confirm this assumption. If suppression of ethanol absorption from the stomach and jejunum is the only factor behind the effect of Shiitake-H, the shape of the curves of acetaldehyde and acetate concentrations would be similar to that of the ethanol concentration with a peak later than 60 min. However, different from the acetaldehyde concentration in the control group, which shifted the peak to 120 min, the Shiitake-H, and Shiitake-N groups had a peak at 60 min and decreased sharply after that. Previously, we reported that the plasma acetaldehyde concentration in the Garlic-H group peaked at 60 min after ethanol administration. Therefore, lentinic acid may have a similar function to ACSO.

On the other hand, the acetate concentration in all the groups was almost the same as that in the control group up to 180 min, and that in the Shiitake-H and Shiitake-N groups decreased sharply at 360 min. These results indicate the acceleration of ethanol metabolism in the stomach and liver. Therefore, alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) activities were measured.

After ethanol is metabolized to acetaldehyde by ADH, acetaldehyde is metabolized to acetate by two major isoforms of ALDH—ALDH1 in cytosol and ALDH2 in mitochondria. About half of Asian people do not possess ALDH2, so they are more susceptible to the effects of ethanol than Caucasians, causing flushing and other vasomotor symptoms after alcohol intake. 6-MSITC in wasabi increases ALDH activity through the induction of mitochondrial ALDH2 expression but not cytosolic ALDH1A1 in HepG2 cells. In vivo, ACSO in garlic increased the activities of mitochondrial ALDH but not cytosolic ALDH in the liver. On the other hand, sulforaphane increased cytosolic and mitochondrial ALDH activities in the liver. Similarly, the actions of cytosolic ADH and cytosolic and mitochondrial ALDH were significantly higher in the Shiitake-H and Shiitake-N groups than in the control group. These results indicate that some components besides lentinic acid in Shiitake-H and Shiitake-N may contribute to accelerating ADH and ALDH activities in the liver.

In addition, Shiitake-H also increased the activities of cytosolic ADH and cytosolic and mitochondrial ALDH in the stomach. These findings suggest that the increase in the activities of ALDH would be principally responsible for the fast metabolism of acetaldehyde by lentinic acid in the Shiitake-H.

The residual ethanol concentration in the stomach was higher in the Shiitake-H group than in the control and Shiitake-N groups. Still, there was no significant difference in acetaldehyde concentrations among all groups. If ADH works on the surface of the stomach, acetaldehyde concentration will increase in the Shiitake-H group. Therefore, this result indicates that ADH would work after absorbed ethanol from the stomach surface. The differences between Shiitake-H and Shiitake-N in the residual ethanol concentration and the activities of ADH and ALDH in the stomach support the assumption that lentinic acid is a functional component in shiitake that suppress ethanol absorption from the gut. As sulfur-containing compounds such as ACSO, sulforaphane, and 6-MSITC are reported to regulate the activities of ALDH, sulfur atoms in lentinic acid may play an important role in exerting the function.

Nuclear factor erythroid 2-related factor (Nrf2) is a transcription factor that up-regulates a diverse array of antioxidant genes. In this regard, ALDH expression was under the major control of Nrf2, the administration of sulforaphane increased ALDH activity in broccoli, and the blood acetaldehyde concentration was decreased. Furthermore, 6-MSITC in wasabi was reported to increase ALDH activity and induce mitochondrial ALDH2 through the Nrf2/ARE pathway in vitro. ACSO also activated the Nrf2/ARE way, increasing ADH and ALDH activities. Lentinic acid likely affects the Keap1/Nrf2/ARE pathway and accelerates acetaldehyde metabolism in the liver. On the other hand, in the stomach, the activities of cytosolic ADH and cytosolic and mitochondrial ALDH were significantly increased in the Shiitake-H group compared with the control and Shiitake-N groups. Therefore, the significant suppression of the elevation in ethanol concentration after oral administration of Shiitake-H with an ethanol solution would be attributed to the acceleration of ethanol metabolism and suppression of ethanol absorption from the gut.

There are several limitations in the present study. First, we did not directly examine whether lentinic acid affected ethanol absorption and metabolism because of its difficulty with purification and synthesis. Second, the single oral administration of Shiitake-H and ethanol would represent acute alcoholism. Therefore, further studies should be carried out to examine the preventive effect of Shiitake-H on chronic alcoholism.