What enzyme is responsible for the initial step in milk digestion in humans?

[harini07]

Homework Statement

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 (No, not renin) D) pepsin

Homework Equations

Casein is converted to Paracasein by rennin and then paracasein along with calcium is acted upon by pepsin to break into peptones.

The Attempt at a Solution

I know that rennin is secreated only in infants in humans and the question that i posted here (as given in my national level exam) asks "humans" which baffels me. 'cause i don't get whether they are asking adult humans only or humans in general (where infancy also included). And in a grown up adult, rennin is not secreated and it's function is carried out by pepsin. so to this question, between option C and D which one will be more appropriate (accurate)? PS: it's not a multiple correct answer question but a single correct answer question where a wrong answer will lead to -1 mark. so please help me with this question.

 

[John Morrell]

I'm just going to say that if you are free to ask people on a forum this question, you could probably just look it up on Wikipedia or some other website, and you'd probably have a very good answer much more quickly. And if you're taking a test and a wrong answer will result in a loss of points, why the heck are you posting on a forum to get the right answer? Shouldn't you be, I don't know, testing your own knowledge?

If there is some other situation I apologize for the criticism, it just strikes me as an odd situation.

 

[harini07]

I'm just going to say that if you are free to ask people on a forum this question, you could probably just look it up on Wikipedia or some other website, and you'd probably have a very good answer much more quickly. And if you're taking a test and a wrong answer will result in a loss of points, why the heck are you posting on a forum to get the right answer? Shouldn't you be, I don't know, testing your own knowledge?

If there is some other situation I apologize for the criticism, it just strikes me as an odd situation.

How do you say that i didn't look up wikipedia? I have been pondering over dozen of web links on digestion of milk which is getting me confused a lot more.No, i didn't asked this question in this forum to get a quick answer but the most accurate or appropriate answer from the experts here. and this question did not appear in the test that i was taking but in the past year paper which i was practicing for my upcoming test (of course which will test my knowledge eventually). I also stated above my attempts at the solution and where i thought it didn't hit me right, which i guess is my current knowledge of this question. After surfing too much on web over loads of articles on "digestion of milk", you find that you had spent so much hours of your self study time just for this single question and that too in vain (as you can't arrive at the conclusion), what would you do dear @John Morrell ? If i were you, i could ask my teacher but alas I'm studying on my own sans teachers far away from college. so asking here to get the answer doesn't seem odd to me PS: your apology is accepted. And at the end the question is still unanswered.:)

 

[John Morrell]

Yeah, sorry I misunderstood the situation. It sounded like you were saying that you were like taking a test at that moment, so I was kind of confused. It sounds like the real problem is that the question is vaguely worded, I would go with pepsin.

 

[harini07]

Yeah, sorry I misunderstood the situation. It sounded like you were saying that you were like taking a test at that moment, so I was kind of confused. It sounds like the real problem is that the question is vaguely worded, I would go with pepsin.

Would you mind me explaining how?

 

[Ygggdrasil]

According to wikipedia, humans do not produce rennin.

 

[harini07]

According to wikipedia, humans do not produce rennin.

*Adult humans don't. As I had already stated humans do secrete rennin but in infancy and rennin is the most important enzyme especially during the intial few steps as it coagulates milk protein into paracaseinate. Pepsin (the other dude who helps in milk protein digestion) comes to play (and can) only after this. My query is since in adults rennin is not produced, I was told that pepsin carries out the process but which( to my little knowledge) seems bizarre as the milk protein digestion doesn't happen without its coagulation (and rennin is needed here). This question vaguely states 'humans' and asks 'intial step' , this all together confused me in deciding the answer. http://googleweblight.com/i?u=http:...igestion-of-milk.html&grqid=vpIK7Ve3&hl=en-IN I looked upto this article which was of good help but still pepsin why? rennin why not? and humans who? :/

 

[Ygggdrasil]

Did you read the wikipedia article?

Chymosin is produced by ruminant animals in the lining of the abomasum. Chymosin is produced by gastric chief cells in young ruminants and some other newborn animals[2] to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption. Some other non-ruminant species, including pigs, cats, and seals, produce it.[3]

A research[4] found a chymosin-like enzime in some human infants but another[5] did not. Humans have a pseudogene for chymosin that does not generate a protein, found on chromosome 1.[3][6] Humans have other proteins to digest milk, such as pepsin and lipase.[7]:262

The article expresses skepticism of the claim of a rennin-like enzyme expressed in human infants.

 

[harini07]

Did you read the wikipedia article?The article expresses skepticism of the claim of a rennin-like enzyme expressed in human infants.

Um...So it is chymosin pseudogene and not the real chymosin?! This seems to nullify whatever I had read for the past 2 days about milk protein digestion. So pepsin should be the answer? As rennin doesn't exist or pseudo exist in humans. PS: I have read from my textbook that rennin coagulates casein and then only pepsin can act upon it.is that wrong? What actually then happens in the digestion of milk protein in humans (be it be adult or infants) also in the link that I quoted (had you checked it?) Says rennin is found in human infants (ibid: Harper's review of physiological chemistry-pg 177). Is that wrong?

 

[Ygggdrasil]

Given that the source you link seems fairly old (most of the references are from the 1950s and 60s) and it's part of a collection entitled "Why We Should Not Eat Animal Products In Any Form," it does not seem like the most reliable source. More recent articles on the topic suggest no chymosin in human infants. For example:

All vertebrates in which the age dependent expression of gastric proteases has been investigated show a characteristic developmental pattern. Neonatal proteases which show partial immunochemical identity with calf chymosin have been observed in several species. The amino acid sequence of lamb chymosin shows 94% of identity with that of calf chymosin. Chymosins from pig and cat show about 85% and 75% of identity with calf chymosin. Chicken embryonic pepsinogen appears to be more related to calf chymosin than to other gastric proteases. A pseudo-gene for a chymosin-like protease from man has been identified. But a functional, human chymosin-like neonatal protease has not been identified. The possible physiological significance of chymosin and the clotting of milk are discussed.

https://www.ncbi.nlm.nih.gov/pubmed/1455182

(Of course, because you cannot really scientifically prove a negative statement, you won't find any articles that can outright disprove the existence of chymosin production in human infants). Do you have a more reliable source talking about chymosin in humans? What does your course material (e.g. your assigned textbook for the course) say on the matter?

 

[harini07]

 

[harini07]

This is from my reference book and those 2 pics from my textbook.

 

[Ygggdrasil]

Given that your course material says chymosin is present in infants, then I agree that it's unclear what you should answer for the question. I would probably go with pepin as the answer, but perhaps you should ask your instructor for clarification.

 

[harini07]

Given that your course material says chymosin is present in infants, then I agree that it's unclear what you should answer for the question. I would probably go with pepin as the answer, but perhaps you should ask your instructor for clarification.

The key has pepsin as answer. But casein is not the only milk protein right? And there are alpha lactalbumin, immunoglobulin to name a few and these milk proteins don't need rennin(do they?). So by that fact, can pepsin be the answer as milk protein digestion is not just of casein but all those together!

 

[harini07]

The key has pepsin as answer. But casein is not the only milk protein right? And there are alpha lactalbumin, immunoglobulin to name a few and these milk proteins don't need rennin(do they?). So by that fact, can pepsin be the answer as milk protein digestion is not just of casein but all those together!

Also my text material saying 'chymosin is present in infants '- is right? Because in the wiki link that you shared quotes 'chymosin pseudogene is present in humans not the real chymosin itself'. Which is right? My text material or the wiki?

 

[Ygggdrasil]

Also my text material saying 'chymosin is present in infants '- is right? Because in the wiki link that you shared quotes 'chymosin pseudogene is present in humans not the real chymosin itself'. Which is right? My text material or the wiki?

Based on the evidence I've read, I would say chymosin is not produced in human infants. The papers that claimed to find chymosin-like enzymes in infant gastric juices are quite old and used very outdated techniques. Rigorous proof of the presence of chymosin in infants would require identifying the gene encoding the chymosin-like enzyme, and no such gene seems to exist in the human genome. Because the human genome has been sequenced, it is unlikely we have missed finding this human chymosin gene.

 

[harini07]

Based on the evidence I've read, I would say chymosin is not produced in human infants. The papers that claimed to find chymosin-like enzymes in infant gastric juices are quite old and used very outdated techniques. Rigorous proof of the presence of chymosin in infants would require identifying the gene encoding the chymosin-like enzyme, and no such gene seems to exist in the human genome. Because the human genome has been sequenced, it is unlikely we have missed finding this human chymosin gene.

I will then unanimously go for pepsin then(Mind note for myself: the textbooks are not always correct.may be they are outdated) :) Thanks I learned a lot about chymosin which I couldn't have been from my textbook and your help was beyond measure :') there is no rennin in humans, Harini! I will never confuse it with the chymosin pseudo gene.[emoji106]

 

[John Morrell]

I support that decision, though I would say that the way they specify "initial step" makes me think that they are looking for Rennin and are mistaken. To pass the test you have to guess not the right answer but the answer they are looking for. At the same time, I think if you answer Pepsin you could argue pretty effectively if they mark you wrong. If this is a test that you can appeal in, I would definitely go with pepsin and then you can be that genius who wipes out the mis-informed test writers :)

 

[harini07]

" To pass the test you have to guess not the right answer but the answer they are looking for" -Completely agree with this.
" At the same time, I think if you answer Pepsin you could argue pretty effectively if they mark you wrong."
No they don't let us argue :p they just release the key, right or wrong their words are final and have to agree with the board.
"If this is a test that you can appeal in, I would definitely go with pepsin and then you can be that genius who wipes out the mis-informed test writers :) "
Yah! but more than outwitting the test writers, it did allow me to know something new and useful which i won't be otherwise. thanks to the question setters, had they provided the answer as rennin, i wouldn't have bothered and might not have learned more.