Severe liver disease impairs hepatic methionine metabolism, thereby reducing SAMe availability.
Abstract: The Effects of S-Adenosylmethionine on Clinical Pathology and Redox Potential in the Red Blood Cell, Liver, and Bile of Clinically Normal Cats
Sadenosylmethionine (SAMe), a naturally occurring molecule, is an essential metabolite important in hepatocytes. It has a pivotal role in the transmethylation, transsulfuration and aminopropylation pathways, whereby it acts as a donor of methyl, sulfhydryl and aminopropyl groups to metabolic precursors. The transmethylation pathways enable metabolic responses, detoxification reactions, energy utilization, gene transcription and membrane functions that influence growth, adaptive responses and cell signaling. The transsulfuration pathway generates endogenous sulfur and thiolated compounds, such as cysteine, glutathione (GSH) and sulfates that are essential for detoxification and conjugation reactions. This pathway provides systemic thiol availability through hepatic GSH synthesis, which is extremely important for cell biology (e.g. gene transcription, triggering proinflammatory cell signaling and apoptosis). The aminopropylation pathway generates products (methioadenosine and polyamines) that influence cell repair, tissue regeneration, inflammatory cascades and apoptosis. Depletion of hepatic glutathione (GSH) can indirectly cause toxic effects by increasing oxidative stress. Cats are particularly susceptible to erythrocyte oxidant injury, as compared to other species. This is, in part, due to an increased number of vulnerable sulfhydryl bonds on the globin portion of hemoglobin. Up to 5 percent of red blood cells (RBCs) from healthy cats demonstrate Heinz bodies as a result of this susceptibility to oxidative damage and relatively inefficient splenic Heinz body removal. The presence of increased Heinz bodies is common in cats with numerous systemic diseases, and may complicate recovery when treatments deliver drugs with oxidant effects.
It has been previously demonstrated in humans and animal models (other than the cat) that transcriptional down regulation of the normally dominant hepatic isoform of SAMe synthetase impairs production of SAMe from methionine. Although SAMe synthesis can occur in all tissues, up to 80 percent of methionine is catabolized through this pathway in the liver, therefore a majority of transmethylation reactions occur predominantly in the liver. With hepatic insufficiency, methionine is retained, and may therefore contribute to the development of hepatic encephalopathy. Because the liver has a large requirement for GSH, reduced availability of SAMe can have profound effects on one's health and recovery from illness.
The high reactivity of SAMe makes this compound labile, thereby compromising its therapeutic delivery. Synthesis of a stable salt containing the biologically reactive isomer (SÂ´S) and its formulation in an enterically coated tablet is commercially available (Nutramax Laboratories) and has allowed therapeutic use of SAM-e. The exact mechanisms of action have not yet been clearly defined, however its metabolic integration in cell metabolism after oral (PO) administration has been proven in different animal species, including humans. Little information is available regarding its use in cats.
To investigate the enteric availability of a stable bioavailable form of the SÂ´S isomer of SAMe and its influence on routine clinicopathological parameters, redox status of RBCs and liver tissue (concentrations of total GSH [TGSH] and its components of reduced GSH [RGSH] and oxidized GSH [GSSG], and malonaldehyde equivalents expressed as thiobarbiturate reacting substances [TBARS], concentrations of TGSH and total bile acids in gallbladder bile, and RBC osmotic fragility in clinically healthy cats).
Design of the study: Fifteen mature (age 2-6 years) domestic shorthair intact female cats were enrolled in a prospective study. Physical examination, a complete blood count (CBC), serum biochemical profile, urinalysis, 12-hour fasting pre and two-hour post serum bile acid (SBA) concentrations were performed. All cats tested negative for FeLV/FIV. Each cat received 180 mg SAMe tablets (Denosyl SD4®) on an empty stomach, for 113 days. The mean daily dosage was 48 mg/kg PO q24h. This dose was chosen as only enteric-coated 180-mg tablets were available at the time of the study, however, 90 mg tablets are now available. Fasting plasma SAMe concentrations were evaluated 0, 2, 4, 8 and 12 hours after administration on days 0, 29 and 111. A CBC, serum biochemical profile, urinalysis, RBC TGSH, TBARS and SBA were performed on days 0, 14, 28, 56 and 112. RBC osmotic fragility testing was performed on days 0, 28, 56 and 112. Liver biopsies and bile were collected under general anesthesia via laparoscopy 10 days prior to initiation of the study and on day 113. Hepatic concentrations of TGSH, RGSH, GSSG, glycogen, total protein and DNA concentrations and TBARS were evaluated. Total bile acids from bile and TBARS were also evaluated. Quality control measure for the CBC samples consisted of concurrently performing CBCs from six clinically healthy cats and from five cats with regenerative anemia (not associated with Heinz bodies). These cats were obtained from the hospital population. All treatment intervals were compared with baseline (day 0) values, with cats serving as their own controls.
Results: SAMe administered PO significantly increased plasma SAMe concentrations in 78 percent of cats, and peak concentrations usually occurred two to four hours following administration. Chronic administration did not alter peak or cumulative plasma SAMe concentrations and did not cause overt clinical signs of toxicity. The hematocrit was significantly higher than baseline on day 14, and lower than baseline on day 28, although values remained within the normal reference range. No significant changes were noted in the MCV; however, a significantly increased RDW percent developed on days 14, 28 and 56. These findings are consistent with an altered RBC population. A positive and significant influence on RBC TGSH and RGSH was observed. In addition, concentrations of RBC TBARS were significantly decreased (by 21 percent). A significant reduction in osmotic fragility was observed. A significant increase in the concentration of liver protein and liver TGSH (by 35 percent), RGSH and the RGSH:GSSG ratio (69 percent) was found. Concentrations of liver TBARS, which were very low at each sampling interval, were not significantly altered by SAMe administration. Hepatic glycogen concentrations decreased by greater than 50 percent, while TP increased. The median concentrations of total bile acids in gallbladder bile increased significantly from day 0 to day 113, but TGSH concentration in gallbladder bile was not different.
Histological evaluation of baseline liver biopsies failed to show any abnormalities in the majority of cats. However, mild and moderate zone 1 portal inflammation was identified in five asymptomatic cats. Interestingly, liver biopsies collected after day 113 showed a reduction in portal triad inflammation. Retrospectively, no significant differences occurred among hepatic TGSH, RGSH or TBARS concentrations or RGSH: GSSG between cats with and without inflammatory lesions at baseline.
No toxicity was noted with chronic PO administration in healthy cats at a dose of 180 mg per day. SAM-e is enterically available after PO administration in fasted cats, and it appears to produce biological responses. Plasma SAMe concentrations were successfully measured, and a significant increase was demonstrated after dosing, with the highest plasma concentrations observed two to four hours after administration in most cats. In addition, plasma SAMe concentrations were significantly increased at 12 hours, and indicated that higher than endogenous SAMe concentrations may persist for long intervals after PO administration. Although there was an increase in RBC TGSH concentration within the first month of therapy, the authors felt that a change in erythrocyte population dynamics likely occurred secondary to repeated low-volume phlebotomy. Thus, the data is difficult to interpret fully due to the lack of a similarly phlebotomized group of cats that did not receive SAMe. The significant and maximal increase in RBC RGSH on days 14 and 28 may reflect regeneration secondary to phlebotomy (consistent with the increased RDW percent). The decreased concentration of RBC TBARS and increased RBC resilience suggest that chronic SAMe administration may have contributed antioxidant and membrane stabilizing effects in RBCs. Results obtained also suggest that SAMe may serve as a therapeutic means of increasing hepatic cysteine availability in cats with spontaneous liver disease that are associated with subnormal hepatic TGSH and RGSH concentrations.
The authors were able to serendipitously evaluate the potential therapeutic effect of SAMe on nonsuppurative hepatic inflammation in the five asymptomatic cats that showed portal inflammation on the initial liver biopsies. However, due to the fact that all affected cats were treated with SAMe, one cannot conclude with 100 percent certainty that the improvement was due to the SAMe. In addition, the natural history of asymptomatic portal inflammation is not fully understood, and spontaneous resolution may occur.
The absence of a change in gallbladder bile TGSH concentration noted simultaneously with a decline in gallbladder bile total bile acid concentration, and increased hepatic TGSH concentrations is consistent with a bile acid-independent choleretic response. The increased hepatic TP concentration is consistent with SAMe's anabolic influence on metabolism and protein synthesis. However, the specific proteins and the number of proteins were not determined. The suppressive influence of SAMe on liver glycogen concentration was unanticipated, especially when compared to rodents, but, unique differences in cats compared with other mammals may explain the observed variations.
The liver metabolized nearly 50 percent of dietary methionine, and converts approximately 80 percent to SAMe, and uses it for transmethylation reactions. It has been shown that severe liver disease impairs hepatic methionine metabolism, thereby reducing SAMe availability, resulting in a multitude of hepatic and systemic ramifications.
Based upon the study's findings, the administration of an enterically coated SAMe tablet (on an empty stomach) is safe, and yields improvement in RBC markers, hepatic redox status, augmented RBC resilience against osmotic challenge, bile acid-independent choleresis and improved histology in cats with asymptomatic nonsuppurative portal inflammation. SAMe has been used in cats with spontaneous disease and anecdotally appears to be clinically effective, however, further studies are required to determine its effects conclusively and scientifically support its use in cats with spontaneous disease. Although not stated in this particular article, the recommended published dose is 18-20 mg/kg PO (on an empty stomach) per day. Because SAMe is easily oxidized when exposed to air, only foil wrapped preparations should be used. The tablets should not be broken or crushed because the product will be inactivated by gastric acid. The product purity is quite variable from formulation to formulation. As a result, it is advisable to use a product manufactured by reputable companies. Very few side effects are documented, however, this author has rarely seen vomiting, diarrhea and abdominal discomfort (presumably from intestinal cramps) with its administration.
Dr. Carioto earned her DVM from the Ontario Veterinary College (OVC), University of Guelph. She spent a year in small animal general practice in New Jersey before returning to the OVC to do an internship, residency and Doctor of Veterinary Science degree in Internal Medicine. She has been board certified in small animal medicine since 2002. Dr. Carioto is currently the proprietor of a mobile veterinary internal medicine referral service in Montreal, Quebec, Canada.