Acetaldehyde inhibits the enzyme protein tyrosine phosphatase (PTP), possibly by blocking the conserved cysteine (sulfur-containing) residue at the active site. PTPs are a group of enzymes that remove phosphate groups from phosphorylated tyrosine residues on proteins. The unimpeded action of PTPs are a necessary part of the formation/maintenance of epithelial tight junctions. These tight junction structures seal the intestinal walls so that they are impermeable to macromolecules and fluids.
Translation: "acetaldehyde can punch holes in the intestinal wall = leaky gut syndrome"