α‐Mannosidase from rat epididymal fluid is a ligand for phosphomannosyl receptors on the sperm surface

Abstract

This study demonstrates that alpha-mannosidase from rat epididymal fluid is a ligand for phosphomannosyl receptors on the sperm surface. This enzyme was bound to intact epididymal spermatozoa with high affinity and in saturable form, and the binding was inhibited by mannose-6-phosphate but not by phosphorylated derivatives of fructose. Treatment of the enzyme with sodium periodate inhibited the binding of alpha-mannosidase, confirming that a carbohydrate residue is involved in the interaction with spermatozoa. Evidence is also presented that the cation-independent phosphomannosyl receptors are responsible for the interaction with alpha-mannosidase. These findings suggest a new role for extracellular transport mediated by the mannose-6-phosphate receptor.

Castration induces changes in the cation‐dependent mannose‐6‐phosphate receptor in rat epididymis: Possible implications in secretion of lysosomal enzymes

“…To investigate this, we have measured the activity of two acid hydrolases, b-NAG and a-MAN in the epididymal fluid, assuming that most of the enzymes were secreted by the epithelium. Contrary to our expectations, it appeared that the secretion of a-MAN [a ligand bearing M6P; Belmonte et al, 1998] was significantly reduced in the epididymis of castrated rats, while the secretion of b-NAG did not differ between control and castrated rats. This apparent retention of a-MAN may be mediated by an increased level of the CI-MPR in the epididymis of castrated rats, as this receptor has been implicated in normal distribution of acid hydrolases to lysosomes.…”

“…Furthermore, some secreted enzymes bearing M6P bind to MPRs on the sperm surface during transit along the epididymal duct [Barbieri et al, 1995;Belmonte et al, 1998Belmonte et al, , 2000. As a result, we wondered whether MPRs might somehow participate in the secretion of specific acid hydrolases and whether the expression and functionality of these receptors would be affected by hormonal changes.…”

Mannose-6-Phosphate Receptors as a Molecular Indicator of Maturation of Epididymal Sperm

“…A mong the several enzymes existing in the¯uid of the rat cauda epididymis , amannosidas e (a-MA N) has been identi® ed as a ligand for the CI-MPR . The interactio n of this enzyme with sperm does not require bivalent cations, and is partially inhibited by either mannose-6-phosphate or a speci® c anti-CI-MPR antibod y [8]. Other enzymes appear not be related to MPR s. A lthough epididyma l¯uid ligands for the CD-MPR have not yet been found, their existence is inferred from indirect evidence [10].…”

“…It seems that the sperm act as a vehicle to transport part of the secreted lysosomal enzymes to an extraepididyma l environment (Figure 1a). This hypothesis is supported by the fact that the near neutral epididyma l pH [45] may favor the enzyme± sperm interaction but not the catalytic activity of the enzymes [4, 8,76]. Subsequently, since the female reproductive tract provides an optimal pH for acid hydrolas e activity [43,65], it is possible that the enzyme may act there to modify glycoprotein s on the oocyte, or the sperm itself, during the capacitation process prior to fertilization .…”

“…It is possible that some of the enzymes secreted by the epididymis may act in the organ itself, such as arylsulfatase, which appeared predominant ly as a soluble enzyme in the¯uid, consistent with a role in sperm maturation . However, some of the secreted enzymes, such as b -GA L, b -NA G, and a-MA N, become transiently bound to the sperm as they are secreted [4, 8,76,77]. This localizatio n may favor transport of the enzymes via spermatozoa to anothe r site.…”