WBR0979

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::Sorbitol concentration is measured in hepatocytes of normal rats and mutant rats with glucose intolerance. The aim of the experiment is to establish the association between the blood concentration of glucose and the concentration of sorbitol in hepatocytes. Shown below is a figure that illustrates sorbitol metabolism. Which compounds correspond to the cofactors X and Y, respectively? ]]
Answer A	AnswerA::X=NADPH and Y=NAD+
Answer A Explanation	AnswerAExp::NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism.
Answer B	AnswerB::X=NAD+ and Y=NADPH
Answer B Explanation	AnswerBExp::While both NAD+ and NADPH are needed for sorbitol metabolism, X corresponds to NADPH and Y corresponds to NAD+, and the order by which the enzymes and their cofactors function in the sorbitol pathway is important.
Answer C	AnswerC::X=ATP and Y=NAD+
Answer C Explanation	AnswerCExp::NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism. ATP is not a cofactor in either reaction.
Answer D	AnswerD::X=NADPH and Y=ADP
Answer D Explanation	AnswerDExp::NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism. ADP is not a cofactor in either reaction.
Answer E	AnswerE::X=NAD+ and Y=ATP
Answer E Explanation	AnswerEExp::NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism. ATP is not a cofactor in either reaction.
Right Answer	RightAnswer::A
Explanation	[[Explanation::NADPH is the reduced form of NADP+ whereas NAD+ is the oxidized form of NADH. Both NADPH and NAD+ are cofactors are electron carriers and donors in the redox reactions of sorbitol metabolism. Glucose is converted to sorbitol by the action of aldose reductase (requires NADPH), and sorbitol is then converted to fructose by the action of sorbitol dehydrogenase (requires NAD+). While some cells are lacking sorbitol dehydrogenase (cells of the retina, Schwann cells, and kidney cells), other cells (e.g. hepatocytes and cells of the ovaries and seminal vesicles) have both enzymes to metabolize glucose into fructose. Below is a figure that illustrates the metabolism of sorbitol Educational Objective: NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism. References: First Aid 2014 page 107]]
Approved	Approved::Yes
Keyword	WBRKeyword::NADPH, WBRKeyword::NAD+, WBRKeyword::Fructose, WBRKeyword::Cofactor, WBRKeyword::Sorbitol, WBRKeyword::Sorbitol metabolism, WBRKeyword::Sorbitol pathway, WBRKeyword::Diabetes
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::