Melamine : Meaning and Toxicity

Melamine is an organic base and a trimer of cyanamide, with a 1,3,5-triazine skeleton. Like cyanamide, it contains 66% nitrogen by mass and, if mixed with resins, has fire retardant properties due to its release of nitrogen gas when burned or charred, and has several other industrial uses. Melamine is also a metabolite of cyromazine, a pesticide. It is formed in the body of mammals who have ingested cyromazine. It has been reported that cyromazine can also be converted to melamine in plants.
Melamine combines with cyanuric acid to form melamine cyanurate, which has been implicated in the Chinese protein export contaminations.
Toxicity
Melamine by itself is nontoxic in low doses, but when combined with cyanuric acid it can cause fatal kidney stones due to the formation of an insoluble melamine cyanurate. Melamine is described as being "Harmful if swallowed, inhaled or absorbed through the skin. Chronic exposure may cause cancer or reproductive damage. Eye, skin and respiratory irritant.” However, the toxic dose is on a par with common table salt with an LD50 of more than 3 grams per kilogram of bodyweight. FDA scientists explained that when melamine and cyanuric acid are absorbed into the bloodstream, they concentrate and interact in the urine-filled renal microtubules, then crystallize and form large numbers of round, yellow crystals, which in turn block and damage the renal cells that line the tubes, causing the kidneys to malfunction.
Acute toxicity
Melamine is reported to have an oral LD50 of 3248 mg/kg based on rat data. It is also an irritant when inhaled or in contact with the skin or eyes. The reported dermal LD50 is >1000 mg/kg for rabbits. In a 1945 study, large doses of melamine were given orally to rats, rabbits and dogs with "no significant toxic effects" observed.
A study by USSR researchers in the 1980s suggested that melamine cyanurate, commonly used as a fire retardant[17], could be more toxic than either melamine or cyanuric acid alone.[18] For rats and mice, the reported LD50 for melamine cyanurate was 4.1 g/kg (given inside the stomach) and 3.5 g/kg (via inhalation), compared to 6.0 and 4.3 g/kg for melamine and 7.7 and 3.4 g/kg for cyanuric acid, respectively.
A toxicology study conducted after recalls of contaminated pet food concluded that the combination of melamine and cyanuric acid in diet does lead to acute renal failure in cats.
Chronic toxicity
Ingestion of melamine may lead to reproductive damage, or bladder or kidney stones, which can lead to bladder cancer.
A study in 1953 reported that dogs fed 3% melamine for a year had the following changes in their urine: reduced specific gravity, increased output, melamine crystalluria, and protein and occult blood.
A survey commissioned by the American Association of Veterinary Laboratory Diagnosticians suggested that crystals formed in the kidneys melamine combined with cyanuric acid, "don't dissolve easily. They go away slowly, if at all, so there is the potential for chronic toxicity." (source : wikipedia.org)