Which enzyme produced from the secretor gene controls the production of H substance (Type I precursor)?

L-fucosyltransferase (found in the saliva of secretors) is the Enzyme Produced from the Secretor Gene that Controls H Substance Production

The enzyme α-1,2-fucosyltransferase (L-fucosyltransferase) is the enzyme produced from the secretor gene (FUT2) that controls the production of H substance (Type I precursor). 1

Understanding the Secretor Gene and H Substance Production

The secretor gene (FUT2) encodes for a specific enzyme called α-1,2-fucosyltransferase, which is responsible for the production of H substance in secretory fluids like saliva. This enzyme catalyzes the transfer of L-fucose in an alpha-configuration to the 2-position of beta-D-galactopyranosyl units, creating the H antigen structure 1.

Key characteristics of this enzyme include:

• It is specifically found in the saliva of secretor individuals
• It is encoded by the FUT2 gene
• It transfers L-fucose to create H substance, which serves as a precursor for ABH blood group substances in secretions

Biochemical Evidence Supporting L-fucosyltransferase

The biochemical evidence clearly demonstrates that the secretor gene (FUT2) is a structural gene encoding a specific fucosyltransferase:

• Studies have shown that the secretor gene encodes α-1,2-fucosyltransferase, which has been purified from human milk and other secretions 2
• This enzyme differs from the plasma-derived fucosyltransferase (encoded by the H gene) in several biochemical properties:
  • Different pH-activity profiles
  • Different thermal stability
  • Different substrate affinities for type 1 and type 2 chain acceptors 2

Molecular Genetics of Secretor Status

Secretor status is determined by the FUT2 gene, with non-secretors having various polymorphisms that result in inactive enzymes:

• The most common polymorphism causing non-secretor status is the 428G→A mutation (found in 88.9% of non-secretors in one study) 3
• Other polymorphisms like 739G→A and 839T→C result in almost inactive enzymes 3
• These mutations prevent the production of functional α-1,2-fucosyltransferase, resulting in the absence of H substance in secretions

Clinical Significance

The secretor status determined by FUT2 and its encoded enzyme has important clinical implications:

• Secretor status affects susceptibility to certain infections, as H substance and related fucosylated carbohydrates can serve as ligands for pathogens 3
• In premature infants, non-secretor and low secretor status is associated with increased mortality, necrotizing enterocolitis, and gram-negative sepsis 4
• The enzyme's activity in producing H substance has implications for blood group antigen expression in secretions

Distinguishing Features from Other Enzymes

The L-fucosyltransferase (α-1,2-fucosyltransferase) encoded by the secretor gene has distinct properties:

• It has higher affinity for type 1 chain acceptors compared to type 2 chain acceptors, though the difference may be small in some tissues 5
• It differs from the H gene-encoded plasma fucosyltransferase in divalent cation requirements and other biochemical properties 5
• It is specifically expressed in secretory tissues rather than hematopoietic tissues

In conclusion, the enzyme produced from the secretor gene that controls the production of H substance (Type I precursor) is L-fucosyltransferase (α-1,2-fucosyltransferase), making option A the correct answer.