Naturally occurring sialic acids can be O-acetylated at any one of four hydroxyl groups, located at position C-4, -7, -8, and -9. This modification, which is found in nearly all animals expressing sialic acids and certain bacteria, is known to be involved in regulating a variety of biological events. One of the more important processes that appear to be heavily influenced by O-acetylation is cancer development. The present review outlines some of the more recent advances towards understanding the role and regulation of sialic acid O-acetylation in human colorectal cancer, basalioma and melanoma. Even though great strides have been made towards identifying and characterising the biological role of O-acetylated sialic acids, detailed information concerning the transferase activity responsible for this modification remains vague. The 7(9)-O- and 4-O-specific acetyltransferases were identified in bovine and horse submandibular glands, respectively, over 30 years ago, however despite the efforts of a number of groups these enzymes have stubbornly escaped purification and cloning, remaining elusive. Here we will attempt to summarise the available data gathered over several decades, as well as detailing recent advances towards understanding the mechanism and regulation of this enigmatic enzyme.
Amylolytic abzymes are a new type of catalytically active human antibodies that have been found recently. Various immunoglobulins from the sera of patients with autoimmune diseases and human milk were found to possess amylolytic activity, which is expressed in their ability to hydrolyze α-(1, 4)-D-glucosyl linkages of maltooligosaccharides, starch, glycogen, and several artificial substrates. Pure IgM fractions isolated from several tens of analyzed patients with clinically definite diagnoses of multiple sclerosis (MS) and systemic lupus erythematosus (SLE) had approximately three orders of magnitude higher specific amylolytic activity than those for healthy donors. Average values for the specific amylolytic activity of IgGs and sIgAs from human milk were five times less than of IgMs from autoimmune patients. Strict criteria were used to prove that the amylolytic activity of abzymes was their intrinsic property and was not due to any enzyme contamination. Fab fragments derived from IgM and IgG fractions of human abzymes displayed the same level of amylolytic activity. Values for Michaelis constants KM in the abzyme-mediated hydrolysis of different α-D-maltooligosaccharides and α-D-maltooligosaccharides with chromogenic and fluorescent label on reducing end were in the range of 1-2 to 0.01mM. Abzyme fractions from different donors demonstrated catalytic heterogeneity in Michaelis-Menten parameters and different modes of action in the hydrolysis of natural and artificial substrates. One part of amylolytic immunoglobulins exhibited exo-amylase action when others have also α-glucosidase activity yielding glucose and capable of cleaving pnitrophenyl α-D-glucopyranoside. Enzymatic properties of all tested amylolytic abzymes distinguished from those of putative human α-amylases. None of the IgM, IgG, and sIgA samples investigated showed transglycosylating ability.
Carba-sugars, carbocyclic analogues of true sugars, be-long to a family of pseudo-sugars, which is currently attracting great interest among researchers in glycochemistry and glycobiology. The structures of carba-sugars very much resemble those of hexo- and pentopyranoses, but its reactivity and stability in vivo differs enormously from those of reducing sugars, mainly because it lacks the aldehyde or keto group. In particular, both natural and synthetic carba-glycosylamines have been shown to possess interesting biological properties in vivo as chemically-stable mimics of true glycosylamines. Moreover, several biologically interesting carba-oligosaccharides bonded by way of N- or O-linkage were designed and prepared on the basis of its unhydrolyzable features in vivo. Therefore, studies on carba-sugars will become one of the major plans for development of biologically active carbohydrate mimics as well as useful research tools for glycobiology.