Table 1: The active pepsin isoenzymes in man.

Pepsin isoenzymeProportion of total pepsinSubstrate: pH optimum rangeMolecular
weight (Da)

1<5%Haemoglobin: 1.9
Collagen: 3.0
Mucin: 2.0
43810The proportion of pepsin 1 rises to 23% in DU
Mucolytic pepsin
Complexed with carbohydrate

2<1%Haemoglobin: 2.139950

380%Haemoglobin: 2.4–3.137150Pepsin 3 is a complex of isoenzymes. The assay measures their combined effects.
Haemoglobin: 3.23A is structurally similar to 3B but is phosphorylated

5 (Gastricsin)6-7%Haemoglobin: 2.0–3.6 (maximum at 3.2) Mucin: 3.5–531620Stable up to pH 7.3

“Pepsin 4” is a complex of pepsin and an inhibiting peptide, hence, it does not appear in the list of active pepsin isoenzymes.
“Pepsin 6” is the remnant of a zymogen, in all probability pepsinogen, hence it too does not feature in the isoenzyme list.
“Zone 7” was found to be a cathepsin.
References [2125].