Acid phosphatase localization in the equatorial region of human lenses

18 kDa protein tyrosine phosphatase in the ocular lens

Protein tyrosyl phosphorylation and dephosphorylation play essential roles in regulating cellular events such as proliferation and differentiation, and their involvement in the lens development and transparency is also suggested. The level of tyrosine phosphorylation in a given protein is regulated by the opposing actions of protein-tyrosine kinases (Tyr kinases) and protein-tyrosine phosphatases (TyrPases). Recent studies have revealed that some Tyr kinases, such as platelet-derived growth factor receptor and fibroblast growth factor receptor, are present in the lens, however, little is known about TyrPases in the lens. In this study, we found a 18 kDa protein tyrosine phosphatase (18 kDa TyrPase) predominantly present in the ocular lens of various animals. We purified the phosphatase from the lens of chick embryo and characterized its activity.Phosphatase activity was determined in chick embryo, mouse, rabbit and bovine lenses using p -nitrophenyl phosphate (p NPP) as substrate. All lenses examined dephosphorylated p NPP under acidic conditions, and a large portion of the activity resided in a low molecular weight protein, ca. 18 kDa, following high-resolution gel permeation column chromatography. The brain and liver showed high dephosphorylation activities, but most of their activity was present in high molecular weight fractions, unlike that in the lens. The 18 kDa phosphatase was purified from the lens of 17 day old chick embryos to near-homogeneity with two-step rapid chromatography. This phosphatase showed strict substrate specificity for phosphotyrosine and phosphotyrosyl peptides, suggesting that it was a kind of protein tyrosine phosphatases (TyrPases). Several known inhibitors of TyrPases, such as SH blockers, vanadate and phenylarsine oxide, strongly inhibited the enzyme activity. The molecular weight, substrate specificity, and responses to various inhibitors and activators coincide well with those reported for the low molecular weight protein tyrosine phosphatase (LMW-TyrPase), belonging to the TyrPase superfamily. These results suggest that the 18 kDa phosphatase found in the lens is a LMW-TyrPase. The 18 kDa TyrPase is the predominant phosphatase in the ocular lens. It may be involved in regulation of lens cell proliferation, differentiation and/or lens transparency.

Biochemical and histochemical evidence for lysosomal proteases in rodent lenses

The lysosomal proteases cathepsin B and dipeptidyl peptidase II (DPP II) were identified biochemically in the supernatant fraction of rat and guinea-pig lens homogenates, using as substrates CBZ-Arg-Arg-4-methylcoumarin-7-amide (AMC) (5 microM, pH 6.0, 37 degrees C for cathepsin B, and Lys-Ala-AMC or Lys-Pro-AMC (5 microM, pH 5.5, 37 degrees C) for DPP II. Cathepsin B-like activity was totally inhibited by the specific inhibitor lepeptin (0.1 mM); while DPP-II-like activity was partially inhibited by Tris (10 mM) or puromycin (0.1 mM), which are specific for DPP II, and by phenylmethylsulphonyl fluoride (PMS-F), an inhibitor of serine class proteases. DPP-II-like activity also was demonstrated histochemically. The histochemical preparations consisted of cryostat sections (6 micron) of glutaraldehyde-fixed rat lenses, incubated in medium containing cacodylic acid-NaOH (0.05 M), Lys-Pro-4-methoxy-2-naphthylamide (MNA) or Lys-Ala-MNA (1 mM), and 0.1% w:v Fast Blue B salt (pH 5.5, 37 degrees C). Control sections were treated with medium in which the substrates were replaced by Lys-Ser-MNA (1 mM), which is not acted upon by DPP II; or medium containing PMS-F (1 mM). Sections treated with experimental medium containing Lys-Pro-MNA showed intense enzymatic activity throughout the epithelium and bow region. In the bow region and superficial cortex activity was mostly granular, suggesting a lysosomal localization. Histochemical activity was reduced partially by inclusion of PMS-F, and totally by substituting Lys-Ser-MNA for Lys-Pro-MNA.