D-Sucrose and Glycogen are the primary sources of D-fructose synthesis.

D-Sucrose is hydrolyzed to D-fructose and D-Glucose by Maltase-glucoamylase (MGAM) and Sucrase-isomaltase (SUIS) [1]. Ketohexokinase (KHK) phosphorylates D-fructose to D-fructose- 1-phosphate [2].

Glycogen is hydrolyzed to Dextran, alpha-D-glucosyl-(1-4)-beta-D-glucose and D-Glucose by Amylase, alpha 2B (AMYC), Amylase, alpha 2A (AMYP) and Amylase, alpha 1C (AMYS) [3], [4]. SUIS then catalyses the formation of alpha-D-glucosyl-(1-4)-beta-D-glucose to D-Glucose [1]. Aldo-keto reductase family 1, member B1 and B10 (AK1BA and ALDR) promote the synthesis of sorbitol from D-Glucose [5], [6]. Sorbitol is then converted to D-fructose by Sorbitol dehydrogenase (DHSO) [7].

Galactose mutarotase (GALM) mediates the conversion of alpha-D-Glucose to beta-D-glucose [8].

beta-D-glucose and D-fructose- 1-phosphate further participate in D-fructose metabolism.

1. Robayo-Torres CC, Quezada-Calvillo R, Nichols BL
   Disaccharide digestion: clinical and molecular aspects. Clinical gastroenterology and hepatology : the official clinical practice journal of the American Gastroenterological Association 2006 Mar;4(3):276-87
2. Asipu A, Hayward BE, O'Reilly J, Bonthron DT
   Properties of normal and mutant recombinant human ketohexokinases and implications for the pathogenesis of essential fructosuria. Diabetes 2003 Sep;52(9):2426-32
3. Groot PC, Bleeker MJ, Pronk JC, Arwert F, Mager WH, Planta RJ, Eriksson AW, Frants RR
   The human alpha-amylase multigene family consists of haplotypes with variable numbers of genes. Genomics 1989 Jul;5(1):29-42
4. Hokari S, Miura K, Koyama I, Kobayashi M, Komine S, Komoda T
   A restriction endonuclease assay for expression of human alpha-amylase isozymes. Clinica chimica acta; international journal of clinical chemistry 2002 Aug;322(1-2):113-6
5. Cao D, Fan ST, Chung SS
   Identification and characterization of a novel human aldose reductase-like gene. The Journal of biological chemistry 1998 May 8;273(19):11429-35
6. O'connor T, Ireland LS, Harrison DJ, Hayes JD
   Major differences exist in the function and tissue-specific expression of human aflatoxin B1 aldehyde reductase and the principal human aldo-keto reductase AKR1 family members. The Biochemical journal 1999 Oct 15;343 Pt 2:487-504
7. El-Kabbani O, Darmanin C, Chung RP
   Sorbitol dehydrogenase: structure, function and ligand design. Current medicinal chemistry 2004 Feb;11(4):465-76
8. Timson DJ, Reece RJ
   Identification and characterisation of human aldose 1-epimerase. FEBS letters 2003 May 22;543(1-3):21-4