甲硫胺酸
**甲硫胺酸 (Methionine)** 是一種含硫的必需胺基酸，在獸醫學中主要用作**尿液酸化劑**和營養補充劑。 * **尿液酸化：** 過去常被用於降低尿液 pH 值，以溶解並預防**磷酸銨鎂結石 (struvite)**。 * **臨床要點：** 在小動物臨床上，其用途已大半被專業處方泌尿道飲食（如 s/d, c/d）所取代，這些飲食能有效控制尿液 pH 值而無需額外補充藥物。 * **抗脂肪肝劑：** 作為抗脂肪肝劑，可預防或糾正因缺乏膽鹼引起的脂肪肝。 * **大型動物用途：** 在豬和家禽中作為營養補充劑；用於治療牛的酮症；並被宣稱可用於治療馬和牛的蹄葉炎（理論上可提供雙硫鍵基質以維持蹄骨結合），但缺乏確切的療效證據。
作用機制: Methionine exerts its effects through several metabolic pathways: * **Urine Acidification:** Methionine is metabolized in the liver, where its sulfur component is oxidized to **sulfate**. This sulfate is excreted by the kidneys as **sulfuric acid** → effectively lowering urine pH. * **Lipotropic Action:** Methionine supplies methyl groups to **ethanolamine** → forms **choline**, which is essential for lipid transport and metabolism in the liver, preventing hepatic lipidosis. * **Antioxidant & Structural Precursor:** Two molecules of methionine convert to one molecule of **cysteine**, a critical precursor for **glutathione** (a major endogenous antioxidant) and essential for disulfide bond formation in structural proteins (like keratin in hooves).
各物種劑量
- Urine acidification · 1000-1500 mg per day given in the food once daily · PO · q24h · Use if diet and antimicrobials do not reduce pH.
- Urine acidification · 0.2-1 gram PO once daily · PO · q24h
- General / Ketosis · 20-30 grams PO · PO
- Laminitis (purported) · 22 mg/kg PO once daily for one week; then 11 mg/kg PO once daily for 1 week; then 5.5 mg/kg PO once daily for one week · PO · q24h · 3 weeks
- Senecio-induced liver damage · 12.5 grams IV in one liter saline/dextrose solution · IV · Once
- Urine acidification (struvite dissolution) · 100 mg/kg PO q12h · PO · q12h · Safe and effective in dissolving presumed infection-induced struvite uroliths in combination with an appropriate anti-microbial agent without using a struvite dissolution diet.
- Struvite dissolution therapy (if diet and antimicrobials do not result in acid urine) · 0.2-1 gram PO q8h · PO · q8h
劑量為合格獸醫專業人員的臨床參考。請務必對照最新藥品仿單及個別病患確認。
給藥途徑
禁忌症
- Renal failure
- Pancreatic disease
- Hepatic insufficiency
- Preexisting acidosis
- Oxalate or urate calculi
- Kittens
- Pre-existing metabolic acidosis
- Severe liver disease or hepatic encephalopathy
- Urate, cystine, or calcium oxalate urolithiasis
- Young growing animals
- Pregnant or nursing animals
不良反應
- Gastrointestinal distress (nausea, vomiting)
- Heinz-body hemolytic anemia (especially in cats)
- Methemoglobinemia
- Ataxia
- Cyanosis
- Metabolic acidosis
- Gastrointestinal upset (nausea, vomiting, diarrhea)
- Heinz body anemia (especially in cats at high doses)
- Worsening of hepatic encephalopathy in patients with liver disease
藥物相互作用
- Aminoglycosides (gentamicin, amikacin) · Urine acidification may diminish the effectiveness of aminoglycosides in treating bacterial urinary tract infections, as they are more effective in an alkaline medium.
- Erythromycin · Urine acidification may diminish the effectiveness of erythromycin in treating bacterial urinary tract infections.
- Quinidine · Urine acidification may increase the renal excretion of quinidine, potentially lowering its systemic levels.
- Other urinary acidifiers (e.g., ammonium chloride) · Additive acidification leading to severe metabolic acidosis · major
- Urinary acidifying diets · Excessive lowering of urine pH and increased risk of metabolic acidosis · major
監測
- Urine pH (goal of therapy is typically ≤6.5)
- Blood pH (if signs of toxicity or metabolic acidosis are present)
- CBC in cats (if exhibiting signs of toxicity, to check for Heinz-body anemia)
- Urine pH
- Complete Blood Count (CBC) to monitor for Heinz bodies, especially in cats
- Acid-base status (blood gases) if overdosage is suspected
- Liver function
過量
**Feline Toxicity:** Cats are particularly sensitive to methionine overdose. * When administered at a dose of 2 grams orally per day to mature cats, it caused **anorexia, methemoglobinemia, Heinz body formation (with resultant hemolytic anemia), ataxia, and cyanosis**. * **Kittens:** Methionine may be highly toxic to kittens who consume other cats' food to which methionine has been added. **General Toxicity:** Metabolic acidosis can occur with overdoses in any species, particularly when used in combination with an acidifying diet. No specific information was located on the treatment of methionine overdosage; treatment should be symptomatic and supportive (e.g., correcting acid-base imbalances).
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