Endogenous biotin can be found in which tissues?

• A. Brain and Heart
• B. Liver and Kidney
• C. Lung and Spleen
• D. Muscle and Skin

Answer: (B)

Endogenous biotin is most abundant in tissues with high metabolic activity and many biotin-dependent enzymes. The liver and kidney fit this, because they house abundant biotin-dependent carboxylases (such as pyruvate carboxylase and acetyl-CoA carboxylase) and are central to biotin metabolism and reabsorption. As a result, these organs have higher baseline biotin levels than other tissues, making them the best choice for where endogenous biotin is found. In practical immunohistochemistry using avidin-biotin detection, this higher biotin content can lead to background staining, so a biotin-blocking step or a non-biotin detection method is often used. Other tissues may contain some biotin, but at lower levels, which is why liver and kidney are the correct focus here.