The authors report careful and sign& cant studies of the synovial fluid concentrations of sialic acid, a term which is applied to a group of related compounds first isolated from saliva. The nature of such compounds is reviewed.Since available methods for determining sialic acid are not spec&, two colorimetric procedures were applied. A critical discussion of methodology is presented.Increased concentrations of sialic acid were observed in both the synovial fluids and sera of rheumatoid patients.Le autores reporta meticulose e signscative studios in re le concentration-in le fluido synovial-del si-appellate acido sialic. Iste termino es applicate a un gruppo de inter-relationate compositos prim0 isolate ab specimens de saliva. Le natura de tal compositos es passate in revista.Viste gue le nunc disponibile methodos pro determinar acido sialic non es specific, duo technicas colorimetric esseva applicate. Es presentate un discussion critic del methodologia.Augmentate concentrationes de acido sialic esseva observate in le fluidos synovial e in le seros de patientes rheumatoide.HE BIOLOGIC IMPORTANCE of a group of compounds designated by T the class name sialic acid is becoming increasingly evident. The name is derived from saliva, the source of the first member of the family, which in 1936 was isolated by Blix from bovine submaxillary mucin.'S2 The sialic acid group is composed of N-acetyl, N-glycolyl, and N, 0-diacetyl derivatives of the same compound, now known as neuraminic a~i d .~?~ The substituent groups vary with species of animal.2 The sialic acid isolated from human serum proteins and other human materials is N-acetylneuraminic acid, but the presence of other sialic acids or the loss of substituent groups during isolation is notThe N-acetylneuraminic acid is also known as ovine or 0-sialic acid.Sialic acid has not been investigated previously in synovial fluid although it, or neuraminic acid, has been found in saliva, serum, urine, cerebrospinal fluid, milk and cyst fluids. Derivatives of neuraminic acid have also been isolated from gangliosides, liver, meconium and red blood c e k 5 The pres-
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