Which of the following is seen in low insulin: glucagon ratio?

Correct Answer: B. Activation of hormone-sensitive lipase

A low insulin:glucagon ratio defines a catabolic state (fasting, starvation, or uncontrolled diabetes). In this metabolic environment, glucagon dominates and insulin is suppressed. Glucagon activates the cAMP-PKA cascade, which phosphorylates and inactivates acetyl-CoA carboxylase (blocking fatty acid synthesis) and simultaneously phosphorylates and activates hormone-sensitive lipase (HSL) in adipose tissue. HSL catalyzes the hydrolysis of triglycerides into free fatty acids and glycerol, mobilizing stored energy. This is the hallmark lipolytic response to low insulin:glucagon ratio. In contrast, a high insulin:glucagon ratio (fed state) suppresses HSL through dephosphorylation and promotes anabolic pathways. The Indian clinical context is critical: patients with uncontrolled type 1 diabetes (very low insulin, high glucagon) exhibit severe lipolysis, leading to ketoacidosis from excessive fatty acid oxidation. HSL activation is therefore the discriminating feature of the catabolic, low insulin:glucagon state and is directly regulated by the glucagon-PKA axis.

Why the other options are wrong

A. Activation of phosphofructokinase 1 — PFK-1 is a glycolytic enzyme activated by high insulin and high AMP (fed/energy-rich state). In low insulin:glucagon ratio (catabolic state), PFK-1 is inhibited by elevated ATP, citrate, and low AMP. Glucagon suppresses glycolysis to spare glucose for gluconeogenesis and ketogenesis. This is the opposite of what occurs in low insulin:glucagon ratio. C. Activation of lipoprotein lipase — Lipoprotein lipase (LPL) is activated in the fed state (high insulin:glucagon ratio) to facilitate uptake of circulating triglycerides from chylomicrons and VLDL into adipose tissue for storage. In low insulin:glucagon ratio, LPL is suppressed and HSL is activated instead to mobilize stored fat, not store it. LPL is insulin-dependent; low insulin means low LPL activity. D. Activation of glycogen synthase — Glycogen synthase is activated by high insulin and dephosphorylation (fed state). In low insulin:glucagon ratio, glycogen synthase is phosphorylated and inactivated by PKA (activated by glucagon), promoting glycogen breakdown via phosphorylase kinase instead. This is an anabolic enzyme suppressed in catabolic states.

High-Yield Facts

Mnemonics

Fed vs Fasted Lipid Metabolism FED = Fatty acid synthesis (ACC active), Lipoprotein lipase active, Fat storage. FASTED = Free fatty acid release (HSL active), Fatty acid oxidation, Ketogenesis. Low insulin:glucagon = FASTED state → HSL activation. PKA Phosphorylation Rule In catabolic state, PKA phosphorylates: HSL (activate), Glycogen synthase (inactivate), Acetyl-CoA carboxylase (inactivate). Remember: PKA = Phosphorylates for Activation of lipolysis, Inactivation of lipogenesis.

NBE Trap

NBE may pair "low insulin:glucagon ratio" with glycolytic enzyme activation (PFK-1) or anabolic pathways (glycogen synthase, LPL) because students confuse "low insulin" with "low energy" and expect energy-producing pathways to activate. The trap is forgetting that glucagon dominates in low insulin:glucagon ratio, driving lipolysis, not glycolysis.

Clinical Pearl

In a fasting Indian patient or uncontrolled type 1 diabetes (very low insulin, high glucagon), HSL activation causes rapid lipolysis → elevated free fatty acids → hepatic ketogenesis → diabetic ketoacidosis (DKA). This is why DKA patients have high serum ketones and FFA despite hyperglycemia—the low insulin:glucagon ratio drives uncontrolled lipolysis.

_Reference: Lehninger Principles of Biochemistry (Ch. 23: Lipid Biosynthesis); KD Tripathi Pharmacology (Ch. 31: Insulin & Glucagon); Guyton & Hall Physiology (Ch. 78: Insulin, Glucagon)_