You already know enzymes catalyze the same reaction over and over—but the USMLE twist is that different versions of the “same” enzyme can tell you where damage happened. That’s the whole game with isozymes: same function, different tissue distribution (and often different subunits), which makes them clinically useful “where did it happen?” markers.
The 5-second rule for isozymes (what to say out loud on test day)
Isozymes = same reaction, different tissues (and different subunits) → use them to localize injury.
If a vignette gives you enzyme patterns and asks which tissue or which condition, think isozyme distribution.
The visual mnemonic: “H & M make the heart move”
Picture two big tissues holding enzyme “badges”:
- H = Heart
- M = Muscle
Then map the classic isozymes by their H/M subunits:
LDH and CK are the classic “subunit isozymes” you can localize.
5-second table: the only isozymes most USMLE questions want
LDH (lactate dehydrogenase) — “H in Heart, M in Muscle”
LDH catalyzes: pyruvate lactate (NADH/NAD)
| Isozyme | Subunits | “Where it lives” (high-yield) | Board-style association |
|---|---|---|---|
| LDH-1 | H | Heart, RBCs | MI (LDH-1 rises), LDH “flip” |
| LDH-5 | M | Liver, skeletal muscle | Hepatic/skeletal muscle injury |
One-liner mnemonic: LDH-1 = “Heart is #1” (H). LDH-5 = “Muscle-heavy” (M).
Classic test phrase: LDH-1 > LDH-2 = “LDH flip” after MI (older marker, but still tested conceptually).
CK (creatine kinase) — “MB = Myo(B)cardium”
CK catalyzes: creatine + ATP phosphocreatine + ADP (energy buffering)
| Isozyme | Subunits | “Where it lives” (high-yield) | Board-style association |
|---|---|---|---|
| CK-MM | M + M | Skeletal muscle | Rhabdo, myositis, trauma |
| CK-MB | M + B | Cardiac muscle | MI (rises early, falls early) |
| CK-BB | B + B | Brain | CNS injury (less common clinically) |
One-liner mnemonic: CK-MB = “M”uscle + “B”lood pump = heart.
Timing (Step 2–style):
- CK-MB rises within hours after MI and returns to normal in ~2–3 days → useful for reinfarction (a new bump after it normalized).
The “5-second rule” for real vignettes: how to answer fast
If they say “Chest pain + enzyme pattern”
- CK-MB up → supports myocardial injury, and helps with reinfarction timing
- LDH-1 > LDH-2 (flip) → classic older MI marker concept
If they say “Muscle pain + dark urine”
- Think skeletal muscle breakdown → CK-MM (and myoglobin—different marker, but often paired)
If they say “Hepatitis / liver injury”
- LDH can point broadly, but liver is more LDH-5 than LDH-1
Ultra–high-yield clarifiers (common traps)
- Isozymes do not mean “different reactions.” They catalyze the same reaction.
- They differ in tissue distribution and often in subunit composition (like LDH and CK).
- Modern practice uses troponin for MI, but USMLE still loves CK-MB/LDH patterns as physiology/biochem localization questions.
Pocket recap (shareable)
- Isozymes = same reaction, different tissues → localize damage
- LDH-1 = H = Heart/RBC
- LDH-5 = M = Liver/Skeletal muscle
- CK-MB = heart, CK-MM = skeletal, CK-BB = brain
- LDH flip (LDH-1 > LDH-2) = classic MI association
- CK-MB normalizes fast → reinfarction clue