Sulfonamides serum protein, binding

The serum-protein binding ability, which varies between animals and is also influenced by the disease state of the animal, will also determine the free diffusible concentration. This, in turn, will have an effect on the elimination of drug residues as well as on their penetration in eggs or milk. This effect will be more pronounced for drugs with a higher tendency for protein binding such as sulfonamides, doxycycline, and cloxacillin (47). 

Sulfaphenazole (684) and sulfazamet (685) are both examples of relatively short acting sulfonamides (B-80MI40406) and their antibacterial activity has been tested against Escherichia coli, the former being more effective than the latter. Sulfaphenazole also displaces sulfonyl ureas from protein binding sites on human serum albumin and consequently increases the concentration of the free (active) drug and produces a more intense reaction that may result in hypoglycemia. 

Sulfonamides are rather slowly absorbed with peak blood levels 2-6 h after oral intake. Intravenous preparations are sometimes used with comatose patients. Sulfonamides are distributed throughout the body, including the CNS. Binding to serum proteins varies from 20% to 90%. Several sulfonamides are acetylated in the liver followed by excretion in the urine. Soluble sulfonamides are eliminated by glomerular filtration. 

Diaminopyrimidines are weak bases. Peak plasma concentrations are reached early and diaminopyrimidines are soon found in high concentrations in tissues. In fact, the tissue concentrations are often higher than the concentrations in serum. When inflammation is present, trimethoprim levels in the CSF may reach 50% of the plasma concentrations. CSF concentrations of pyrimethamine are 25-50% of the plasma concentrations. The Vd for trimethoprim and pyrimethamine is 1.51/kg in horses. The protein binding of trimethoprim is moderate (50%). There is no protein-binding interaction between the sulfonamides and the diaminopyrimidines. 

The polymeric nature of the AgSD complex renders it almost insoluble in water and organic solvents and this property is in fact critical to its activity. The MIC of the Ag ion in culture media is of the order of 1—20 g/mL, due to binding, and thus inactivation, to serum proteins, etc. The MIC of free sulfadiazene as its sodium salt (NaSD) is 500—1000 fig/mL while that of the complex (AgSD) is 5—100 //g/mL, an approximately 5-fold increase over that of the free Ag ion. Strains resistant to free sodium sulfadiazene or silver are sensitive to the silver salt, and the sulfonamide antagonist p-aminobenzoic acid (PABA) does not affect efficacy. 

Albumin is a protein polypeptide. Albumin has a molecular weight of 66.5 kDa and is the most abundant plasma protein, which is present in the concentration of 35-50 g/L in human serum and is synthesized in the liver. Human serum albumin (HSA) has a half-life of 19 days. It acts as a solubilizing agent for long chain fatty acids and is therefore essential for the transport and metabolism of lipids. It binds very well to penicillins, sulfonamides, indole compounds, benzodiazepines, copper, and nickel in a specific and calcium and zinc in a relatively nonspecific manner. It is responsible for osmotic pressure of the blood. 

Serum albumin is the most abundant plasma protein (3.5-5.5%) and is responsible for the binding and transport of various metabolically and pharmacologically active molecules, e.g., bilirubin, uric acid, vitamin C, acetylcholine, cholinesterase, adenosine, aureomycin, barbiturate, Chloromycetin, digitonin, fatty acids, atabrine, neosilversalvarsan, penicillin, salicylate,/ -aminosalicylate, sulfonamide, streptomycin, acid dyes, histamine, triiodothyronine, and thyroxine (Bennhold, 1962 Putnam, 1975). Moreover, albumin tightly binds various metal ions, e.g., Zn (Giroux,... 

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