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Question

Posted on: March 6 2013

A prescription of a dermatologist for a beverage with cystine intended for a child aged 1.5 years

L-methionine 3.75 g L-cystine 3.75 g raspberries Syrup AD 150 cc 2 x 1 teaspoon per day < br/> L-cystine is not soluble in water and alcohol! May I replace it with L-cysteine since L-cystine is a dimere form of L-cysteine? If yes, do I double the amount of L-cysteine? Should this dosage be for a child aged 1.5 years?

Answer

physico-chemical properties of cystine:

  • soluble in water up to 0.01%
  • soluble in acid and alkali with salt formation
  • pKs1 (Carboxyl) = 1, pKs2 (Carboxyl) = 2.1, pKs3 (Amin) = 8.0, pKs4 = (Amin) 8.7
  • pH of solution = 5.02
  • dissolution in the Alcalisch middle

We therefore possess the ability to either acids or add base. Use of cystine solutions: mainly as eye drops 3.6%.

eye Drops formulation 360 mg in a small amount of water dissolving after adding 176 mg NaOH being 2.2 g of a 8.0% NaOH solution neutralization with 6.8 g of an isotonic boric acid solution (1.9%) Up to pH = 9.7 Supplement with physiologically serum up to 10 ml.

But for an oral preparation this composition does not seem so suitable. Personally, I would prefer to dissolve cystine with diluted acid. But the solubility of cystine at pH = 1 is equal to 170 mg/L. Consequently, there is only possibility of suspending cystine after prior grinding. I don't think that using cysteine HCl is a good solution considering this product in solution converts to cystine, which then crystallizes.