homework - Substrates of cytochemical reactions in this immunostaining

Expression of extracellular protein Laminin 9 alpha-4 chain in human skeletal muscle.
Indirect immunostaining with HRP immunostain marker. Ob.x40.

I have unsuccessfully searched NCBI -database, JSTOR and other major Biology databases for an answer. This suggests to me that I do not understand what is going on.

1. What are the substrates of above cytochemical reactions for the given immunostaining above?

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I need to give a simpler question where I apparently know the reaction exactly, since it is possible that people cannot answer the above question.

Mitochondria in Hep-2 cell line cells. Cytochemical test for mitochondria specific enzyme NAHD dehydrogenase. Ob.x40.

2. What is the substrate of the cytochemical reaction?

My answer:

The reaction of NADH dehydrogenase is:

NADH + H⁺ + CoQ → NAD⁺ + CoQH₂

Substrate: CoQ