The exopolyphosphatase activity of wheat phytase toward polyP is unusual in some aspects. Despite the fact that wheat phytase exhibits the highest activity toward phytate and
pNPP at acidic pH (4.5 to 5.0), it has no activity at pH 7.5 [
11], The exopolyphosphatase activity of this enzyme toward polyP75 and polyP1150 was observed at pH 5.2 and 7.5 (
Figure 1). On the other hand, calf intestinal alkaline phosphatase previously known as a sole higher eukaryotic-derived enzyme with exopolyphosphatase activity could not degrade polyP15 or polyP75 at pH below 6.0 at all and 90% of its maximal activity was lost at pH 7 to 7.5 [
13]. Intriguingly, the catalytic action mode of exopolyphosphatases against polyP substrates at acidic and neutral pH may be different because numbers of Pi residues within substrates required for binding to the enzyme at acidic and neutral pH are 2 and 3, respectively [
13]. In particular, wheat phytase was kinetically favorable in degrading long-chain polyP because its overall catalytic efficiency (V
max/K
m) for polyP1150 was 1.46-fold higher than that for polyP75 (
Table 1). However, values of K
m and V
max for polyP75 of wheat phytase were 100-fold higher and 250-fold lower, respectively, than those for polyP77 (almost identical to polyP75 in length) of calf intestinal alkaline phosphatase [
13]. Thus, the kinetic performance for the exopolyphosphatase activity of wheat phytase toward medium-chain polyP seems to be poor. The exopolyphosphatase activity of wheat phytase toward polyP75 was almost unaffected by the presence of divalent metal ions such as Mg
2+, Ni
2+, Co
2+, or Mn
2+ (
Figure 2). Its activity toward polyP1150 was inhibited by increasing the concentration of Ni
2+, Co
2+, and Mg
2+ (
Figure 3). Additionally, EDTA, a well-known metal remover, showed negligible effect on the exopolyphosphatase activity of wheat phytase (
Figures 2,
3), suggesting that divalent metal ions could not act as cofactors for its catalytic activity. In previous studies, the exopolyphosphatase activity of inorganic pyrophosphatase from cattle tick,
Rhipicephalus microplus, relied on Mg
2+ [
9] whereas activities of calf intestinal alkaline phosphatase and soluble pyrophosphatases from protozoa such as
Trypanosoma brucei and
Leishmania amazonesis required Zn
2+ as cofactors [
13–
15]. The monophosphate esterase activity of wheat phytase toward
pNPP was inhibited by the presence of polyP75 or polyP1150 in a dose-dependent manner (
Figures 4,
5). Thus, it appears that the active site for polyP substrates of wheat phytase is consistent with that for
pNPP [
13]. Similar observation was also made with calf intestinal alkaline phosphatase, displaying competitive inhibitory fashion of
pNPP-degrading activity by polyP [
13].
So far, chickens have been regarded as the primary reservoir of
Campylobacter jejuni and
Salmonella typhimurium infection [
7,
16]. Moreover, long-chain polyP molecules like polyP1150 secreted by these enteric bacteria can aggravate inflammation in hosts such as human and bird [
17], leading to intestinal mucosal damage, inflammatory diarrhea, and enteric fever [
16,
18]. Such situation can severely decrease the productivity of poultry industry and pose public health concern [
7]. In conclusion, wheat phytase with an unexpected exopolyphosphatase activity has potential as a therapeutic tool and a next-generational feed additive for controlling long-chain polyP-induced inappropriate inflammation from
Campylobacter jejuni and
Salmonella typhimurium infection in public health and animal husbandry because wheat phytase is a relatively safe and endogenous strategy that can overcome drawbacks caused by resistance to antibiotics [
6,
19] and prejudice and mistrust against the use of foreign microbial enzymes [
20].