NDA (I) 2017 General Ability Test · Question 66 of 149

Correct — A, Renin. Renin is released by the juxtaglomerular cells packed into the wall of the afferent arteriole of each nephron. They fire when blood flow through the kidney drops, when less sodium reaches the distal tubule, or when sympathetic nerves stimulate them. Once in the blood, renin cleaves angiotensinogen — a plasma protein manufactured by the liver — to give angiotensin I. That single step is the rate-limiting one for the whole renin-angiotensin-aldosterone system, so the kidney effectively sets the pace of the body's main blood-pressure-raising cascade.

• (b)Nitrogenase — Nitrogenase is a bacterial enzyme that converts atmospheric nitrogen into ammonia, most familiarly inside the root nodules of legumes. It has no role in the human kidney and belongs to a plant-and-microbe topic altogether.
• (c)Hydrolase — Hydrolase names an entire class of enzymes that split bonds using water. Renin itself is a member of that class, being an aspartic protease, so the label is true but far too broad — it identifies no particular kidney secretion, and the question asks for the enzyme by name.
• (d)Mono-oxygenase — Mono-oxygenases insert a single oxygen atom into their substrate; the liver's cytochrome P450 family is the standard example, used in drug metabolism. They do not act on angiotensinogen.

The renin-angiotensin-aldosterone system is the body's main long-run blood-pressure control loop. The kidney senses low perfusion and releases renin; renin converts liver-made angiotensinogen to angiotensin I; angiotensin-converting enzyme, mostly on the lining of lung blood vessels, trims angiotensin I to angiotensin II; angiotensin II narrows blood vessels, raises sodium reabsorption in the proximal tubule, increases sympathetic outflow and triggers the release of aldosterone and vasopressin. Aldosterone in turn makes the collecting duct hold on to more sodium and water.

The stem hands over the substrate and the product, so the task is only to name the kidney's enzyme. Three of the four options come from unrelated corners of biochemistry, which is a common NDA construction — one precise term surrounded by real but irrelevant enzyme names. If the term is unfamiliar, the giveaway is that renin sounds like and sits beside the word renal.

• Renin is secreted by the juxtaglomerular cells in the wall of the afferent arteriole of the nephron.
• Angiotensinogen is made by the liver and circulates in plasma; renin cleaves it to angiotensin I, which is itself inactive.
• Angiotensin-converting enzyme, largely in the pulmonary circulation, turns angiotensin I into the active angiotensin II.
• Angiotensin II raises blood pressure directly and also by stimulating aldosterone, which retains sodium and water.

• Confusing renin, the kidney enzyme, with rennin, the milk-curdling enzyme of the stomach of young mammals — the spellings differ by one letter.
• Picking a broad enzyme class such as hydrolase when the question wants a specific named enzyme.

NDA usually gives the gland or organ and asks for its secretion, or gives the secretion and asks what it does — so learn each hormone or enzyme along with its source and its single main action.

• practice — not a real PYQ

  Angiotensin I is converted into the active angiotensin II mainly by an enzyme found on the vessel lining of which organ?

  • (a)Liver
  • (b)Lungs
  • (c)Spleen
  • (d)Pancreas

  Answer(b) Lungs — angiotensin-converting enzyme sits largely on the endothelium of the pulmonary circulation.

• practice — not a real PYQ

  Which one of the following hormones makes the kidney tubule reabsorb more sodium and water?

  • (a)Insulin
  • (b)Thyroxine
  • (c)Aldosterone
  • (d)Glucagon

  Answer(c) Aldosterone — released from the adrenal cortex under the influence of angiotensin II.