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The intial step in the digestion of milk in humans is carried out by which of the following enzymes?

A) lipase.

B) trypsin.

C) rennin.

D) pepsin.

I'm confused between the C and D option. as it's not specified whether they're asking about adult humans only or humans in general (where infancy also included), which between pepsin or rennin will be the answer for this question?

I know that in infants the milk digestion is initiated by rennin and in case of adults due to its absence it's carried out by pepsin. Which is the correct option for that question?

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Chris Mar 23 '17 at 20:51
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Short answer
My best bet is D)

Background
A) seems incorrect as lipases break down fats. However, one has to realize there quite some fat in milk (up to 3.5% in untreated milk). Digestive lipases in man are secreted by the pancreas into the duodenum.

B) Might be a correct answer, as trypsin is released in the small intestine and...

[Trypsinized] peptide products are further hydrolyzed into amino acids via other proteases, rendering them available for absorption into the blood stream. Tryptic digestion is a necessary [first] step in protein absorption as proteins are generally too large to be absorbed through the lining of the small intestine.

C) Chymosin, known also as rennin, is a proteolytic enzyme synthesized by chief cells in the stomach. Its role is thought to be in digestion is to curdle or coagulate milk in the stomach, a process important in young animals. If milk were not coagulated, it would rapidly flow through the stomach, impairing protein digestion. So this is a enzyme in the stomach and active earliest in the digestive system (answers A and B concern enzymes in the duodenum, which is later down in the digestive tract). However, in humans there is only a chymosin pseudogene present, and since the answer specifically deals with humans, answer C) is incorrect.

D) Pepsin is

...[] one of three principal proteases in the human digestive system, the other two being chymotrypsin and trypsin. [These] enzymes [...] break down dietary proteins into [...] amino acids, which can be readily absorbed by the small intestine. Pepsin [mainly] cleav[es] peptide bonds between hydrophobic and [..] aromatic amino acids [...The other two proteases have different targets].

Pepsin's proenzyme, pepsinogen, is released [...] in the stomach [...], and upon mixing with the [...] gastric juice, [it is] activate[d] to [...] pepsin.

Here we have the adult equivalent of rennin, also active early in the digestive tract, namely the stomach. So D) is the enzyme present in humans (as opposed to C) and active already in the stomach (as opposed to later in the duodenum in answers A and B). So D is the answer.

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  • $\begingroup$ physicsforums.com/index.php?threads/908437 please do check this link.I agree with your short answer but the reason why pepsin seems to be contrary to what I learnt from the link that I quoted.Have a look at that link and come back here again to let me know your views on that :) $\endgroup$ – Harini Mar 31 '17 at 10:02
  • $\begingroup$ I did. physicsforums.com/index.php?threads/908437. Did you? $\endgroup$ – Harini Mar 31 '17 at 14:13
  • $\begingroup$ you say chymosin is present in human infants. But chymosin wiki says humans don't have chymosin.they have chymosin pseudogene. So pepsin is the answer be it be human infants or humans is what I had learnt. $\endgroup$ – Harini Mar 31 '17 at 15:08
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    $\begingroup$ @user129402 - gotcha. This is on humans and they lack the chymsoin gene. Nice thinking! $\endgroup$ – AliceD Mar 31 '17 at 15:15
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    $\begingroup$ @AliceD we should also remember that there are many proteins in milk other than casein, which are digested first by pepsin. $\endgroup$ – JM97 Mar 31 '17 at 18:04

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