All of the following enzymes are present in Leydig cells except?

SJH Nursing Officer - 2019

Biochemistry & Nutrition

Medium

Appeared in: SJH Nursing Officer - 2019

Explanation

21-hydroxylase (CYP21A2) is an enzyme primarily located in the adrenal cortex, not the testes.
Its main function is to catalyze steps in the synthesis of mineralocorticoids (aldosterone) and glucocorticoids (cortisol).
Leydig cells are specialized for producing androgens (testosterone). The absence of 21-hydroxylase is critical for shunting steroid precursors into the testosterone synthesis pathway.

Option A: CYP (Cytochrome P450) is a large superfamily of enzymes. Leydig cells contain numerous CYP enzymes that are essential for testosterone synthesis, including CYP11A1 (cholesterol side-chain cleavage) and CYP17A1 (17-alpha hydroxylase/17,20 lyase).
Option B: 17-alpha hydroxylase is a crucial enzyme (part of CYP17A1) in the synthesis of both cortisol and androgens. It is present and active in Leydig cells to produce precursors for testosterone.
Option C: 17, 20 lyase activity (also part of CYP17A1) is the rate-limiting and committing step for androgen production. It is highly expressed in Leydig cells to direct steroid synthesis towards testosterone.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Enzymatic pathway of steroidogenesis in Leydig cells as background academic context rather than a clinical decision trigger.
Understanding these specific enzyme pathways is fundamental for nurses to comprehend the pathophysiology of various endocrine disorders, especially Congenital Adrenal Hyperplasia (CAH) and disorders of sexual development.
A nurse caring for a newborn with ambiguous genitalia or an infant in an adrenal crisis must understand that an enzyme defect, like 21-hydroxylase deficiency, is a likely cause, guiding assessment and immediate care.
This knowledge is also relevant in adult health, particularly in the context of infertility and hormonal imbalances.

How to Approach the Question

First, identify the core of the question. It's an 'EXCEPT' question, meaning you need to find the one option that does not belong.
Next, recall the primary function of the cell type mentioned: Leydig cells produce testosterone (an androgen).
Consider the major classes of steroid hormones: androgens (testosterone), glucocorticoids (cortisol), and mineralocorticoids (aldosterone).
Evaluate each enzyme option based on its role in these pathways. 17-alpha hydroxylase and 17,20 lyase are essential for making androgens.
Recognize that 21-hydroxylase is the key enzyme for the cortisol and aldosterone pathways, which occur in the adrenal cortex, not the testes.
Conclude that 21-hydroxylase is the enzyme that is absent from Leydig cells, making it the correct answer.

Concept Tested & Keywords

Concept Tested: Enzymatic pathway of steroidogenesis in Leydig cells
Stem keywords: enzymes, Leydig cells
Lead-in keywords: except
Negative lead-in flag: This is a negative-lead question asking you to find the option that does NOT fit.

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E6 Nelson Textbook of Pediatrics(2024) — Volume 2 pp. 1275-1277, 1252-1254