2.2 Detection Enzymes

2.2 Detection Enzymes

Quick Answer: Chromogenic IHC does not show the antibody. It shows a reporter enzyme attached to the detection reagent converting a soluble mixture into a colored precipitate. Horseradish peroxidase (HRP) uses hydrogen peroxide to oxidize donors such as DAB. Alkaline phosphatase (AP), usually calf-intestinal, hydrolyzes naphthol phosphates that then couple to azo dyes. Glucose oxidase is rare, insensitive, and has no mammalian counterpart. Know where endogenous peroxidase and AP live, why paraffin heat knocks down most tissue AP, and why levamisole spares the detection enzyme.

After the primary antibody (and usually a polymer or secondary) has bound, the visible step is enzymatic. The enzyme is not the chromogen. The enzyme catalyzes conversion of a colorless substrate/chromogen mixture into an insoluble colored product at the antigen site. If the enzyme is in the wrong place — endogenous heme in a red cell, leftover AP on a frozen villus — you still get color. That color is not specific IHC.

The three enzymes QIHC still expects you to compare

Modern automated stainers mostly run HRP polymers and, for second colors, AP polymers. PAP (peroxidase-antiperoxidase) and APAAP (alkaline phosphatase-anti-alkaline phosphatase) are unlabeled-antibody ancestors. You may never pipette PAP in 2026, but the outline still treats enzymes as a detection topic, not as a chemistry-analyzer analyte.

HRP and the hydrogen peroxide / DAB reaction

HRP is a heme glycoprotein. In the presence of hydrogen peroxide, it oxidizes electron-donor chromogens. With 3,3'-diaminobenzidine (DAB):

1. Antibody-bound HRP meets H2O2 — the true substrate of the peroxidase reaction.
2. DAB donates electrons and polymerizes.
3. The polymer is a brown, flocculent, alcohol-insoluble precipitate that survives graded alcohols, xylene, and resinous mounting medium.

If you omit H2O2, DAB does not develop. If H2O2 is far too concentrated or left on too long, you can inactivate HRP or chew up morphology. Working levels in the chromogen buffer are typically in the low-millimolar / roughly 0.01–0.03% range — not the ~3% hydrogen peroxide used to block endogenous peroxidase before the antibody. Mixing up those two bottles is a classic blank-slide or burned-tissue event.

Sodium azide in some antibody diluents inhibits peroxidase. Azide is a preservative, not an HRP friend. If a new diluent lot suddenly kills every HRP assay while AP-red assays look fine, read the label before you re-titer the primary.

Do not freeze-thaw enzyme conjugates. Store them as the insert specifies, usually refrigerated. Heat that would be routine for HIER is not something you apply to the HRP bottle.

Endogenous peroxidase: erythrocytes and granulocytes

Human tissue contains heme proteins with peroxidase-like activity. High-yield sites:

• Erythrocytes — hemoglobin. Bloody specimens, hemorrhage, spleen, kidney, decalcified bone with crush artifact.
• Granulocytes — neutrophils and especially eosinophils (myeloperoxidase).
• Some macrophages in inflammatory backgrounds.

If you apply DAB plus H2O2 with no antibody, red cells still turn brown. That is endogenous peroxidase, not CD31, not cytokeratin, not "the polymer is too sensitive." Blocking with hydrogen peroxide (aqueous or methanolic) or a commercial dual endogenous-enzyme block is a later Detection Systems topic; here you need to recognize the enzyme so you know when to switch the reporter to AP instead of drowning the slide in peroxide.

Methanol-peroxide also lyses many red cells, which can clean a bloody paraffin section. It can also harm some epitopes and is not automatically legal on frozen or cytologic material. Choose the block for the specimen, not for habit.

Worked example. A crushed lymph-node smear is full of naked erythrocytes. An HRP-DAB CD3 stain is unreadable because every red cell is brown. The same primary with an AP–Fast Red detection system avoids the heme enzyme. You still must watch endogenous AP if that smear were a frozen ileal biopsy rather than a node touch prep.

Alkaline phosphatase chemistry

Tissue AP and calf-intestinal detection AP hydrolyze phosphate esters. In IHC the usual substrate is a naphthol AS phosphate (AS-MX, AS-BI, and similar). The released naphthol immediately couples to a diazonium salt (Fast Red TR, Fast Blue BB) forming an azo dye at the site. Kits may instead use Permanent Red, New Fuchsin, or BCIP/NBT (blue-purple).

AP wants alkaline pH, often about 8.5–9.5, and likes divalent cations such as Mg2+. Do not expect AP to perform in leftover HRP acetate buffer at pH 5. Do not add levamisole to an HRP-DAB working solution and expect brown to improve — levamisole is not a peroxidase reagent.

APAAP complexes are large. That history is why AP methods were recommended on blood and marrow smears: you could visualize leukocytes without fighting endogenous peroxidase. Polymer AP reagents still earn that niche.

Endogenous AP: intestine, placenta, and frozen tissue

Endogenous alkaline phosphatase is abundant in:

• Small-intestinal brush border
• Placental trophoblast
• Proximal renal tubules, osteoblasts, some bile-ductular reactions, and some leukocytes

Frozen sections, cytology smears, and under-processed tissue keep this activity. Drop AP substrate on a frozen ileum with no antibody and the brush border turns red. That is endogenous AP.

Levamisole

Levamisole (often 1 mM, sometimes 5 mM in the substrate) inhibits most non-intestinal tissue AP isoforms — liver, kidney, bone, leukocyte. Two exam-critical caveats:

1. Intestinal AP is not adequately inhibited by levamisole. Placental AP is also relatively resistant. A frozen small-bowel biopsy can still self-stain.
2. The detection enzyme is calf intestinal AP, so levamisole is chosen because it spares the label while knocking down most host isoforms. Levamisole does not block HRP and does not convert DAB from brown to black.

If the stem says the red brush border is present on the omitted-primary control of frozen gut, levamisole was the wrong or insufficient answer; switch enzymes or interpret with that control in view.

Why FFPE often has little remaining AP

Alkaline phosphatase is heat-labile. Routine paraffin processing exposes tissue to warm alcohols, clearing agents, and molten paraffin commonly near 55–60 °C. That heat, plus fixation, denatures most tissue AP. Consequences:

• Many FFPE IHC labs omit levamisole on ordinary paraffin sections and still have clean AP-red stains.
• Do not assume the same of frozen tissue, imprints, or lightly fixed material.
• Intestine and placenta can still surprise you on paraffin if enzyme survives. A negative-reagent control with AP chromogen on those tissues is not optional.

This is a stability statement, not permission to skip controls. "FFPE has no AP" is too strong; "FFPE often has little remaining AP compared with frozen tissue" is the version that survives a QIHC stem.

Choosing HRP versus AP versus glucose oxidase

Use HRP when you want a permanent brown archive, endogenous peroxidase is blockable, and you are not stacking a second enzyme on the same slide.

Use AP when:

• Endogenous peroxidase is overwhelming (marrow smears, bloody cytology).
• You need a second color in dual IHC (HRP-brown plus AP-red).
• You want to stop fighting heme enzymes rather than escalate peroxide.

AP does not by itself solve biotin background in liver or kidney. That is a biotin-block problem for avidin-biotin methods, not an enzyme-identity problem.

Reach for glucose oxidase only in specialty double-label teaching: no endogenous enzyme to block, but catalytic rate is low, so sensitivity is poor. It is not the routine clinical default and it is not a substitute for a validated HRP polymer on a diagnostic FFPE panel.

Keep the next chapter's blocking recipes in their place. This section's job is to know which enzyme you attached, which endogenous enzyme can impersonate it, and which inhibitor (peroxide versus levamisole) belongs to which impersonator.