Intestinal Digestion and Absorption of Carbs, Proteins and Fats Flashcards
What percentage of nutrients are already absorbed by the time the meal reaches the distal jejunum?
95%
What are some structural and functional features of the duodenum and jejunum that facilitate digestion and absorption?
plicae circularis slow transit of food
villi increase surface area
segmenting contractions facilitate mixing
What are the three general places within the small intestine where enzymatic hydrolysis of nutrients occurs?
- in the lumen with pancreatic enzymes
- at the microvillious membrane with brush border enzymes
- in enterocytes - for the peptides at least
What enzyme starts breaking down sugars first?
amylase
Amylase blocker supplements have been suggested to promote weight loss. WHy don’t they really work?
they’re proteins that just get broken down in the gut
plus we have enzymatic activity ine xcess and undigested carbs can be broken down by bacteria leading to diarrhea and bloating
What alpha-glucosidease inhibit has been used to treat type II diabetes?
acarbose
Amylase products cannot be imediately absorbed and need further cleavage into which 3 absorbable monosaccharides at the brush border?
glucose
fructose and galactose
Describe an effect of end-product inhibition that glucose has?
it can inhibit lactase activity
How is the regulation of brush border hydrolases regulated temporally?
they are typically brokendown after a meal and then resynthesized before the next meal
How are the brush border hydrolases regulated spatially?
they’re located on the villi and in higher concentrations in the proximal intestine than the distal intestine
How do the brush border hydrolases avoid proteolytic cleavage?
they’re heavily glycosylated for protection
What brush border hydrolase is an exception to the idea that htey don’t want to be cleaved?
sucrase-isomaltase is a single polypeptide that containes 2 active sites
tis’ cleaved into two pieces by trypsin
how can you treat lactose intolerance if it’s due to a lack of lactase?
just supplement lactase with lactaid
What transporter takes up glucose form the duodenum and jejunum? What else will it take?
SGLT1
also galactose
note - it’s secondary actieve transport so it requires a los intracellular Na concentration to make the gradient
What transporter takes up fructose form the djodenum and jejunum?
GLUT5
What basolateral transport will send all monosaccharides to the liver?
Glut2
Digestion of proteins is initiated where? By what enzyme?
stomach with pepsin
At what pH is pepsin inactivated?
over 4.5 - so in theory it should only be active in the stomach
What enzymes take over for pepsin?
pancreatic proteases like endopeptidases and carboxypeptidases
What brush border enzyme is required for activation of the pancreatis proteases?
enteropeptidase
Proteins rich in what residue will be resistant to digestion?
proline
and of course anything glycosylated
What is necessary for the apical membrane transporters to take up amino acids?
they are Na-dependent symporters, so you need the basolateral NaK pumps working
What is the main peptide transporter? What does it cotransport peptides with?
PEPT1
cotransports the peptides wiht protons
What supplies the luminal H ion for this?
the apical sodium/H exchanger which puts H into the lumen
supported by the NAK ATPase of course
What happens to the peptides after they’re brought into the cell?
further digested by cytosolic proteases down to individual amino acids
What is special about protein digestion and absorption during the first 6 months of life?
intact proteins can be absorbed by endocytosis - this is the mechanism of passive immunity in infants
What cells will take up proteins form the gut to transfer them to lymphocytes and antigens (mediating a lot of food allergies)?
M cells
Why don’t genetic disorders of apical amino acid transporters result in amino acid deficiencies?
Because you can still get the amino acids from the peptides that are brought into the cell and then further broken down
Most of the dietary lipids are in what form? How about otherse?
90% are triglycerides
10% are cholesterol, phospholipids, and lipid soluble vitamins
What cells secrete gastric lipase to initiate lipolysis?
cheif cells
Gastric lipase is not required, but what population relies on it?
neonates because they have developmental delay of pancreatic enzyme expression
also in people with pancreatic insufficiency
how is pancreatic lipase inhibited by bile acids?
they emulsify the fats and block lipase binding to the fat droplets
What molecule is necessary to bind the bile acids, thus recruiting lipase to cleave the fatty acids?
colipase
What other two pancreatic enzymes contribute to lipid digestion?
cholesterol esterase phospholipase A2 (phospholipids)
Why was olestra added to chips?
it’s a huge fat that’s too big to be absorbed. idea was that it would be effective for weight loss
turned out to be supeer side effecty
What pancreatic lipas einhibitor is used to treat obestiy?
orlistat (xenecal)
Does orlistat work?
it does have a modest effect, but many regain the weight after taken off the drug
does reduce risk of type 2 CM and lowers blood pressure but you get fatty stools, abdominal cramping, diarrhea and vat soluble vitamin deficiencies
