Question: Palmitate uniformly labeled with the radioisotope 3H to a specific activity of 2.5 x 108 cpm (counts per minute; i.e., amount of radioactivity) per micromole of palmitate is added to mitochondria, which oxidize it to acetyl-CoA. All acetyl-CoA is isolated and hydrolyzed to acetate and CoA. The specific activity of the acetate is found to be 1 x 107 cpm per micromole. I.e., there are 1/25th as many radioactive Hydrogens per molecule of acetate as there was per molecule of palmitate. Is this result consistent with the beta- oxidation pathway? Why? What is the final fate of the missing 3H?

Answer: This is consistent with the pathway because most of the hydrogen is lost from the fatty acids in the form of NADH, FADH2, and H+ ions. Following the dehydrogenation, hydration and dehydrogenation reactions of beta-oxidation, only one labeled hydrogen atom on the acetyl-CoA is cleaved in the following thiolysis step. When the palmitate has undergone seven rounds of oxidation, what is produced is 7 acetyl-CoA, each with one labeled hydrogen, and one acetyl-CoA with 3 labeled hydrogens (the last one does not undergo any modification because the seventh round cleaves the 4 carbon chain into two 2-carbon chains). Adding up the label on all the acetyl-CoA molecules, there are 10 labeled hydrogens on the 8 acetyl-CoA molecules. On average, each acetyl-CoA has an average label of 10/8 = 1.2. Comparing the original label (31 hydrogens) with the final label (1.2 hydrogens), the label is reduced by a factor of 31/1.2 = 25. Thus, the label on the acetyl-CoA is 1/25th of the label on the palmitate.

WHERE I NEED HELP…..I’m having trouble understanding this problem. I understand the process of beta oxidation and why 8 Acetyl CoA’s are formed but I’m confused what “radioisoptio 3H” means (where is the radioisotope attatching on the palmitate?) and also the section I italicized in the answer portion. Maybe I don’t fully understand the statement since I’m not sure what hydrogen is labeled and where it is being moved to (and why one acetyl CoA has 3 labeled hydrogens while the rest have only 1?)