Fructosamine-6-phosphates are deglycated by phosphorylation to fructosamine-3,6-bisphosphates catalyzed by fructosamine-3-kinase (FN3K) and/or fructosamine-3-kinase-related-protein (FN3KRP)

Nonenzymatic glycation of proteins and some phospholipids by glucose and other reducing sugars (a.k.a Maillard reaction) is an unavoidable result of the coexistence of these sugars and the affected macromolecules in living systems. The consequences of this process are deleterious both in the intracellular and extracellutar environments as evidenced by the close association between increased nonenzymatic glycation and complications of diabetes. Because of these considerations, we have proposed that the intrinsic toxicity of glucose and other sugars is counteracted in vivo by active deglycation mechanisms including transglycation of Schiff's bases and FN3K-dependent breakdown of fructosamines. While this modified hypothesis is receiving increasing experimental support, several issues regarding glycation/ deglycation remain unresolved. Two such important questions are (a)What is the mechanism whereby products of nonenzymatic glycation derived from glucose-6-phosphate are degraded? (b) What is the role of FN3K-related protein (FN3KRP) that, in spite of similar to 75% identity in amino-acid sequence with FN3K, does not phosphorylate fructosamines? In this paper we propose a resolution of both these quandaries by proposing that fructosamine-6-phosphates are deglycated by phosphorylation to fructosamine-3,6-bisphosphates catalyzed by FN3KRP and/or possibly FN3K. We provide some preliminary evidence in support of this hypothesis and outline experimental approaches for definitive tests of this hypothesis. The potential medical implications of this finding are not clear yet but, if correct, this observation is likely to have a major impact on our understanding of the very basic and hitherto unexplored aspect of glucose metabolism and chemistry in vivo. One can imagine that, at some point in the future, measurement of FN3K/FN3KRP activity may be of diagnostic value in assessing an individual's susceptibility to diabetic complications. Further down the road, one can also envision a gene therapeutic intervention to bolster FN3K/FN3KRP-based antiglycation defenses. (c) 2006 Elsevier Ltd. All rights reserved.