Question

Which of the following do not belong to the category of brush border enzymes?
A. Lactase, nucleotidase
B. Aminopeptidase, sucrase
C. Procarboxypeptidase, steapsin
D. Nucleosidase, maltase​

Answer 1

Hint: The brush border is a complex and extremely plastic organelle essential for intestinal homeostasis and is specialised for the absorption of nutrients. Thousands of densely packed microvilli from the brush border and the area they are situated on, is called a terminal web.

Complete step-by-step anwer:
Succus entericus also known as the intestinal juice is a fluid that is excreted in small quantities in the small intestine. The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells comprise this intestinal juice. It is extremely variable in the constitution and contains different enzymes like maltase, nucleosidases, dipeptidases, enterokinase, invertase, lactase, and lipase and mucus. The nucleases are generated in the pancreas. They are digestive enzymes that break down nucleic acids such as RNA and DNA into nucleotides, which are the building blocks of nucleic acids. When these nucleotides reach the ileum which is the final part of the small intestine, they are further digested into bases, sugars, and phosphates.

Procarboxypeptidase is the other protein-splitting proenzymes found in pancreatic juice. Steapsin is another name for the enzyme lipase. This name is used to describe the lipase enzyme in pancreatic juice. This is one of the digestive enzymes which is liable for the digestion of fat. The enzyme catalyses the breakdown of fats into glycerol and fatty acids. Lactase, Maltase, and Nucleotidase are brush-border enzymes that catalyze the final catabolism of metabolites in the small intestine. Steapsin and Procarboxypeptidase are pancreatic enzymes. Lactase, Maltase, and Nucleotidase are brush-border enzymes that catalyze the final catabolism of metabolites in the small intestine. Steapsin and Procarboxypeptidase are pancreatic enzymes.

Hence the correct answer is option C.

Note: The last step in the digestion of dietary carbohydrates and proteins takes place on the face of small intestinal enterocytes, in close proximity to the transporters which will ferry the resultant sugars and amino acids into the epithelial cells. The enzymes liable for this end stage of digestion are not accessible in the intestinal lumen but tethered as integral membrane proteins in the plasma membrane of the enterocyte. The apical plasma membrane housing these enzymes consist of several microvilli which extend from the cell and form the "brush border". Therefore, the enzymes incorporated in those microvilli are known as brush border enzymes.